A Ma Aeolake in Alacologi N the Moe Cal Analy

Faculty of Sciences Department of Geology and Soil Science

Research Unit Palaeontology

Academic year 2009‐2010

Changes in surface waters: a malacological analysis of a Late Glacial and early Holocene palaeolake in the Moervaartdepression (Belgium).

Lynn Serbruyns

Thesis submitted to obtain the degree of Master in Biology.

Promotor: Prof. Dr. Jacques Verniers Co‐promotor: Prof. Dr. Dirk Van Damme

Faculty of Sciences Department of Geology and Soil Science

Research Unit Palaeontology

Academic year 2009‐2010

Changes in surface waters: a malacological analysis of a Late Glacial and early Holocene palaeolake in the Moervaartdepression (Belgium).

Lynn Serbruyns

Thesis submitted to obtain the degree of Master in Biology.

Promotor: Prof. Dr. Jacques Verniers Co‐promotor: Prof. Dr. Dirk Van Damme Acknowledgements0

First of all, I would like to thank my promoter Prof. Jacques Verniers and Prof. Philippe Crombé for providing me with this interesting subject and for giving me the freedom to further extend the analysis beyond the original boundaries. Thanks to my co-promoter Prof. Dirk Van Damme who I could always contact with questions and who provided me with many articles on the subject. I also want to thank Prof. Keppens for giving me the opportunity to perform the isotope analysis at the VUB, even though technology let us down in the end.

I would like to thank Koen Verhoeven for sacrificing part of his office and for aiding me with the sampling from the trench. Thanks to Mona Court-Picon for the numerous ways in which she helped me during the making of this thesis and for the nice talks.

Thanks to Thomas Verleye and Bert Van Bocxlaer for aiding me with the statistical analysis and for explaining to me how Photoshop and Illustrator work. Without you two it would have taken me much longer and as you both know time is precious when writing a Master thesis. I also want to thank Prof. Achilles Gautier for correcting my English writing.

Special thanks goes toward Elena Dierick and Pieter-Jan Vandermeeren which gave up several days of their weekends and/or holiday to help me collect the samples for the evaluation of the recent malacofauna. Thanks to my family, boyfriend and friends for tolerating the neglect and the numerous conversations about snail over the past year.

Finally, I would like to thank everyone involved in the GOA project and everyone from the Research Unit Palaeontology for their kindness, constructive comments and the several slices of cake and other sweets. Contents 0

ACKNOWLEDGEMENTS

EXTENDED ABSTRACT 1

CHAPTER 1: INTRODUCTION 4

CHAPTER 2: GEOARCHAEOLOGICAL FRAMEWORK 6

2.1 LOCATION AND DEMARCATION OF THE STUDY AREA 6 2.2 GEOLOGICAL TIME FRAME AND CLIMATE 7 2.3. THE FLEMISH VALLEY 8 2.4 THE MOERVAARTDEPRESSION: GEOMORPHOLOGICAL AND HYDROLOGICAL EVOLUTION 10 2.5 PRESENT‐DAY HYDROGRAPHY 12 2.6 HUMAN OCCUPATION HISTORY 12

CHAPTER 3: MOLLUSCAN ECOLOGY 14

3.1 ABIOTIC FACTORS 14 3.1.1 OXYGEN 14 3.1.2 PH AND LIME CONTENT 15 3.1.3 SALINITY 15 3.1.4 CURRENTS AND WAVE EXPOSURE 15 3.1.5 TEMPERATURE 16 3.1.6 WATER DEPTH 16 3.1.7 DROUGHT 16 3.1.8 POLLUTION 17 3.2 BIOTIC FACTORS 17 3.2.1 FOOD 17 3.2.2 ENEMIES 18 3.2.3 COMPETITION 18 3.2.4 LOCOMOTION 18 3.2.5 DISPERSAL MECHANISMS 19 3.2.6 REPRODUCTION 19 3.3 WATER TYPES 20 3.4 TAXONOMIC DESCRIPTION OF THE RECOVERED SPECIES 21

CHAPTER 4: MATERIALS AND METHODS 35

4.1 THE RECENT MALACOFAUNA 35 4.2 THE FOSSIL MALACOFAUNA 37 4.3 STABLE CARBON AND OXYGEN ISOTOPES 41

CHAPTER 5: RESULTS 43

PART I: THE RECENT MALACOFAUNA 43

5.1 THE RECENT SPECIES 43 5.2 STATISTICAL ANALYSIS 44

PART II: THE FOSSIL MALACOFAUNA 46

5.3 THE FOSSIL SPECIES 46 5.4 THE S2 SEQUENCE 47 5.4.1 RELATIVE ABUNDANCE AND MOLLUSCAN ZONATION 47 5.4.1.1 Correspondence Analysis 47 5.4.1.2 Molluscan zonation 48 5.4.2 ABSOLUTE ABUNDANCE 51 5.4.3 SPECIES DIVERSITY 54 5.4.4 ENVIRONMENTAL INDICES 54 5.5 THE S4 SEQUENCE 55 5.5.1 RELATIVE ABUNDANCE AND MOLLUSCAN ZONATION 55 5.5.1.1 Correspondence Analysis 55 5.5.1.2 Molluscan zonation 57 5.5.2 ABSOLUTE ABUNDANCE 59 5.5.3 SPECIES DIVERSITY 61 5.5.4 ENVIRONMENTAL INDICES 62

CHAPTER 6: DISCUSSION 64

6.1 THE RECENT MALACOFAUNA 64 6.2 THE FOSSIL MALACOFAUNA 65 6.2.1 HYPOTHETICAL SCENARIOS OF THE MOERVAARTDEPRESSION DEVELOPMENT 65 6.2.2 MALACOLOGICAL/ENVIRONMENTAL PHASES 67 6.3 COMPARISON WITH MOLLUSCAN EVIDENCE OF ELSEWHERE 70 6.4 COMPARISON OF THE FOSSIL AND RECENT MALACOFAUNA 72 CHAPTER 7: CONCLUSIONS 74

PLATE 1 76

PLATE 2 83

REFERENCES 84

APPENDICES 85

Extended abstract 0

The Moervaartdepression is located in the northern part of the province of East Flanders (NW Belgium), not far from the Dutch border. Nowadays it is a lowland area characterised by meadows and agricultural land, but it used to be a large palaeolake. At the beginning of the Late Glacial, the formation of a large aeolian coversand ridge north of the depression blocked the northwards drainage of several rivers and caused indirectly erosion of broad shallow depressions. These depression were filled up with water due to a rise in the water table and created a series of palaeolakes, among which the Moervaart palaeolake was the largest. Lacustrine sediments were deposited during the Late Glacial and the early Holocene. Eventually, the lake dried out due to a lowering of the water table.

The Moervaartdepression has a rich archaeological and palaeoecological potential and has been the subject of detailed and systematic research. Numerous Final Palaeolithic sites are present in the lowlands of Flanders (NW Belgium), but often these sites are disturbed making the reconstruction of the Late Glacial human recolonisation difficult. According to the latest data, Flanders was probably not occupied until the Federmesser culture appeared (ca. 12,500‐9,000 BC). Comparison with neighbouring countries points towards a delayed recolonisation of Belgium. In spite of former research efforts, the Late Glacial recolonisation process is still not well understood (Crombé & Verbruggen, 2002). Therefore, the University of Ghent set up a multidisciplinary research project (Geconcerteerde Onderzoeksactie, GOA) in April 2008, funded by the Special Research Fund (BOF) and titled “Prehistoric settlement and land‐use systems in Sandy Flanders (NW Belgium): a diachronic and geo‐archaeological approach”. The Research Unit Palaeontology of Ghent University is responsible for the palaeoecological part of the project. It includes the malacological investigation here presented.

None of the past studies paid much attention to the numerous freshwater molluscs present in the sediments. Usually only a list of species is included without any interpretation. Yet, a detailed study of the fossil malacofauna is an important tool to interpret the local environment. Parameters that can be deduced are ambient and water temperature, water depth, size of the water body, hydrographical connections, oxygen content and aquatic vegetation (Davies, 2008). The palaeomalacological investigation aimed at a palaeoenvironment reconstruction and was performed on sediment sequences obtained from a trench in the centre of the Moervaartdepression. The palaeontological study was combined with an investigation of the recent molluscan fauna in the same region. The fossil and recent faunas were compared in order to find out what impact man has had on the fauna since the Late Glacial. Originally the comparison would only include sampling sites of good water quality, according to the Belgian Biotic Index (BBI) calculated by the Vlaamse Milieu

Maatschappij (VMM) in 2007, but since some of the selected sites showed signs of habitat deterioration the study was expanded to include intermediate and poor quality sites. Fossil molluscs were also sampled for stable isotope analysis (δ18O and δ13C), but the planned analysis at the Isotope Ratio Mass Spectrometry (IRMS) laboratory of the Vrije Universiteit Brussel (VUB, Prof. Dr. E. Keppens) were not done because of technical problems.

Information on the ecological preferences of the studied molluscan species is available since they still live today. Various abiotic and biotic factors determine their presence. The main deterministic abiotic factors are oxygen and lime content, salinity and temperature of the water. Other abiotic factors include pH, water movement, water depth and periodic desiccation of the water body. For the study of the recent fauna, man‐induced pollution can be added to the list of abiotic factors. Biotic factors relate to the living molluscs and their activities. Next to food, predation and competition reproduction, dispersion and locomotion play an important role in their distribution.

For the investigation of the recent fauna, rivers and brooks in the catchment of the Moervaart and Zuidlede River were sampled during the summer of 2009. Site selection was based on the “Biologische waterkwaliteit in Vlaanderen 2007” map, created by the VMM. Values of several environmental parameters (e.g. pH, conductivity, nutrients) were also collected from the VMM website. Eleven sites in total were sampled: seven sites of good biological water quality (BBI 7‐8), two of moderate quality (BBI 5‐6) and two of poor quality (BBI 3‐4). The faunal composition of the communities was analysed by means of Correspondence Analysis (CA), followed by environmental fit (envfit) analysis to verify which environmental parameters influence the community composition and whether water quality could be assessed based on molluscs alone. The parameters that demonstrated a significant relation are biological water quality, nitrates, chloride and chemical oxygen demand.

For palaeontological analysis, two sequences (S2 and S4) were sampled from a trench in the deepest part of the palaeolake. Sequence S2 consists of 18 samples over a depth of 136 cm, sequence S4 contains 14 samples over a depth of 164 cm. Per sample, 500 ml of sediment was wet‐sieved: 125 ml was used for the palaeoenvironment reconstruction, the rest for the planned stable isotope analysis. All identifiable shells were counted and relative abundances, biodiversity indices and environmental indices (temperature, size of the water body, amount of water movement and drought) were calculated. A biozonation was created based on relative abundances and checked by Correspondence Analysis (CA). Ten zones were defined in sequence S2 and eight in sequence S4. The temperature, size and movement indices displayed strong simultaneous fluctuations; the drought index showed slightly different fluctuations. Biodiversity increased from the bottom of the profile until a plateau was reached, which lasted throughout most of the later sequence, after which the indices declined again at the top of the profile. Owing to the lack of absolute dating and other proxy data, we created three scenarios for the Moervaartdepression development. Each scenario consists of four periods characterised by drastic changes. Scenario 1 and 2 span a time period from the Pleniglacial/Oldest Dryas to the Preboreal whereas scenario 3 covers the shortest time period, from the Pleniglacial/Oldest Dryas to the Allerød. According to scenario 2, which we prefer, the first period was attributed to the Pleniglacial/Oldest Dryas. During this cold phase, the landscape gradually developed from a terrestrial to an aquatic environment. Simultaneously, the temperature and size of the water body increased and the molluscan community became richer. The second period (Bølling/Allerød) was dominated by Bithynia tentaculata, a frost sensitive species. The environmental

2 indices together with the community composition point towards a large well‐vegetated and well oxygenated lake in a warm climate. Period 3 (Younger Dryas) is again a cold period composed of two drops in temperature separated by a short climate amelioration. During this period, the molluscan community of the Pleniglacial/Oldest Dryas and the Bølling/Allerød is replaced by another community with similar habitat preferences, that is, a preference for well‐vegetated and well oxygenated waters. The forth and last period was attributed to the Preboreal. The gradual re‐ establishment of the community during this warm phase was interrupted by the Preboreal Cold Oscillation, after which the molluscan community did not recover as the lake disappeared. The terrestrial fauna at the top of the profile indicates open marshland or damp grassland and nearby dry grassland.

The second analysis consisted of a comparison between the recent and fossil communities. The malacofaunas resemble each other to a large extend, although some species have disappeared since the Late Glacial while others were found only in the recent communities. The species that disappeared are Myxas glutinosa, a pollution sensitive species, and all Pisidium species except one, Pisidium subtruncatum. Five species were observed only in the recent samples: three of these (Bithynia leachii, Lymnaea palustris, Musculium lacustre) have been recovered from Quaternary deposits elsewhere; the others (Dreissena polymorpha, Physella acuta) were introduced by man.

In general we can conclude that freshwater molluscs prove to be good indicators for the reconstruction of the palaeoenvironment. The comparison of the fossil and recent fauna indicated that molluscs are quite resistant to human‐induced habitat alterations. The investigation of the recent fauna showed that molluscs may be useful indicators for water quality assessment, although further investigation on the subject is necessary.

Introduction 1

The Moervaartdepression has a rich archaeological and palaeoecological potential and has been the subject of detailed and systematic research. Numerous Final Palaeolithic sites are present in the lowlands of Flanders (NW Belgium), but often these sites are disturbed making the reconstruction of the Late Glacial human recolonisation difficult. According to the latest data, Flanders was probably not occupied until the Federmesser culture appeared (ca. 12,500-9,000 BC). Comparison with neighbouring countries points towards a delayed recolonisation of Belgium. In spite of former research efforts, the Late Glacial recolonisation process is still not well understood (Crombé & Verbruggen, 2002). Therefore, the University of Ghent set up a multidisciplinary research project (Geconcerteerde Onderzoeksactie, GOA) in April 2008, funded by the Special Research Fund (BOF) and titled “Prehistoric settlement and land-use systems in Sandy Flanders (NW Belgium): a diachronic and geo-archaeological approach”. The Research Unit Palaeontology of Ghent University is responsible for the palaeoecological part of the project. It includes the malacological investigation here presented.

None of the past studies paid much attention to the numerous freshwater molluscs present in the sediments. Usually only a list of species is included without any interpretation. Yet, a detailed study of the fossil malacofauna is an important tool to interpret the local environment. Parameters that can be deduced are ambient and water temperature, water depth, size of the water basin, hydrographical connections, oxygen content and aquatic vegetation (Davies, 2008). The palaeomalacological investigation aimed at a palaeoenvironment reconstruction and was performed on sediment sequences obtained from a trench in the centre of the Moervaartdepression. The palaeontological study was combined with an investigation of the recent molluscan fauna in the same region. The fossil and recent faunas were compared in order to find out what impact man has had on the fauna since the Late Glacial. Originally the comparison would only include sampling sites of good water quality, according to the Belgian Biotic Index (BBI) calculated by the Vlaamse Milieu Maatschappij (VMM) in 2007, but since some of the selected sites showed signs of habitat deterioration the study was expanded to include intermediate and poor quality sites. Fossil molluscs were also sampled for stable isotope analysis (δ18O and δ13C), but the planned analysis at the Isotope Ratio Mass Spectrometry (IRMS) laboratory of the Vrije Universiteit Brussel (VUB, Prof. Dr. E. Keppens) were not done, because of technical problems.

We start our analysis (chapter 2) by presenting the Moervaartdepression and the time period in which our study is situated, followed by a description of the development, the drainage pattern and the human occupation of the Moervaartdepression. Chapter 3 deals with general molluscan ecology and highlights the various abiotic and biotic factors influencing molluscan communities. In this

4 chapter a short description of the recovered species is also given. Chapter 4 describes the acquisition and the processing of the data. Results are given in chapter 5 and evaluated in chapter 6. The final chapter presents the main conclusions of our study, together with some suggestions for further research.

Geoarchaeological framework 2

2.1 Location and demarcation of the study area

The Moervaartdepression is located in the northern part of the province of East Flanders (NW Belgium), not far from the Dutch border. The area stretches from the canal Gent-Terneuzen in the west to Sint- Niklaas in the east. The northern border is formed by a large coversand complex, known as the coversand ridge of Verrebroek (Stekene) - (Maldegem) Gistel (Heyse, 1979) or “the Great Ridge” (Crombé & Verbruggen, 2002) and consists of aeolian deposits covering the pre-existing topology (Kasse, 2002). In what follows the coversand ridge is addressed as the coversand ridge Maldegem-Stekene for short. The southern delineation is less pronounced, but covers more or less the area between Gent and Lokeren (Fig. 1).

The Moervaartdepression is surrounded by various traditional landscapes. The Zeeuws-Vlaamse polders and the coversand ridge Maldegem-Stekene are located to the north. The Land van Waas lies east of the depression. All the former are part of an area known as Zandig-Vlaanderen (Sandy Flanders) (De Moor, 1963; Jongepier, 2009).

The demarcation of the Moervaartdepression is aided by the clear differences in deposits between the depression and the surrounding areas. Within the Moervaartdepression, the sequence is dominated by clays, peats, marls and sands. The marl is located in the central part of the depression and can be found less than 20cm below the present day surface over an area of 8 by 2 km (Verbruggen, 1971). To the north there is a sharp transition from lacustrine sediment to sandy deposits, originating from the coversand ridge. The southern border is less abrupt. The marl-peat sequence of the depression gradually changes towards sandy deposits with wet loam-sand and wet clay intercalations (Heynderickx, 1982). The sands north of the Moervaartdepression are drier than those on the south side (Jongepier, 2009).

From a purely administrative point of view, the investigated part of the Moervaartdepression covers the municipalities of Moerbeke and Wachtebeke, which are located within the province of East Flanders, in the district of Ghent. The study site for palaeontological analysis is located within the western part of the Moervaartdepression, where the sequence of Late Glacial lake sediments is most complete. For the investigation of the recent malacofauna, a larger area was demarcated, stretching the entire Moervaartdepression. More information about the exact location of the sampling sites is given in chapter 4.

Fig. 1: Location of the Moervaartdepression and the coversand ridge Maldegem-Stekene within NW Belgium with indication of the position of the trench (white dot) (source: Ann Zwertvaegher, unpublished).

2.2 Geological time frame and climate

The Quaternary is accepted to begin 2.588 Ma ago and is divided into the Pleistocene (2.588 Ma – 11.5 ka) and the Holocene (11.5 ka – present). The period is characterised by glacial episodes, that is, cold periods during which continental ice sheets were formed, alternating with warm interglacial phases during which the continental ice sheets retreated. This alternation of cooler and warmer periods lasted throughout the Pleistocene and was caused by variations in solar radiation, the so-called Milankovitch cycles (Hays et al., 1976), and/or changes in changes in ocean circulation (Goslar et al., 2000).

The time frame of interest for our study is the end of the Weichselian glaciation, the so-called Late Glacial and the beginning of the Holocene. The Late Glacial period is characterised by a series of rapid alterations of colder and warmer phases, called interstadials. Three cold interstadials were defined: Dryas I or Oldest Dryas (15.0-13.0 ka BP), Dryas II or Older Dryas (12.0-11.8 ka BP) and Dryas III or Younger Dryas (11.0-10.0 ka BP). These periods are alternating with warmer phases: Bølling (13.0-12.0 ka BP) and Allerød (11.8-11.0 ka BP) (Walker, 1995).

Dating of sediments is generally done in two ways: radiocarbon dating or investigation of the preserved pollen sequence. Both types of investigation have been performed on the Moervaartdepression. The pollen sequence has been investigated by Verbruggen (1971, 1979) and indicated a time frame from the early Oldest Dryas to the second part of the Allerød. Radiocarbon dating was less satisfactory. In 1971, two peat samples were submitted by Verbruggen for 14C analysis which gave the following results: 12065±65 BP (GrN-6376) and 11955±105 BP (GrN-6032(3)). Both indicate the start of the sequence around 12,000 uncal. BP. A second series of 14C-datings was performed on 27 levels. Unfortunately, these datings do not show any time evolution. This problem with radiocarbon dating is generally known and is linked to the large “bulb” in the INTCAL98 curve (Stuiver et al., 1998) around 13,000 cal BC. In the future, the available data will need to be revised with the help of the newer INTCAL04 calibration curve (Crombé, 2005). More 14C-datings will be performed on plant remains in the framework of the GOA project.

2.3 The Flemish Valley

The northern part of Belgium is generally characterised by sandy to light sandy-loamy sediments and is therefore known as Sandy Flanders (Fig. 2). The core area of Sandy Flanders is the Flemish Valley, a Quaternary feature characterised by filled in deep Late Pleistocene thalwege or riverine valleys of the Schelde River Basin in the north of Belgium (De Moor, 1995). The mean height above sea level of the Valley varies between +5m in the north and +15m in the south, making it a relief-poor lowland area. The Flemish Valley spans the region between Maldegem and Stekene, north of Ghent and has a total length of about 50km. From the core area, the valley expands further to the south along the Schelde and Leie River and to the east along the Schelde, Rupel and Dijle River (De Moor & Heyse, 1994).

Fig. 2: Position of the sand deposits of Sandy Flanders with indication of the core area or Flemish Valley (source: http://cartogis.ugent.be/sandy_flanders/).

The incision and filling of the Flemish Valley were complex and occurred in several phases. In general, erosion took place at the beginning of cold and moderate phases, causing deep incision of the Young

Quaternary rivers into the Tertiary substrate. Sedimentation, on the other hand, was typical of cold phases and led to the filling of the thalwege by thick (15-30m) packets of Young Quaternary sediments (Spiers, 1986; De Moor en Heyse, 1994). This process eventually gave rise to the sandy infilling plain we still observe today (De Moor & Heyse, 1994). For more information on the Flemish Valley, we refer the reader to De Moor & Heyse (1978, 1994), Heyse (1979), Kiden (1991, 2006), Van Zeir (2009) and Verbruggen et al. (1991) for more information.

One important event however which needs to be stressed is the evolution of the River Schelde (Fig. 3). Until the beginning of the Late Glacial, the Flemish Valley had been characterised by a braided river system (De Moor en Heyse, 1994), that is, a network of small channels separated by small and often temporary islands or braid bars. Fluvial supply came from the (south)west and the (south)east, drainage happened in north-western direction. At the end of the Pleniglacial, the situation changed drastically. The formation of the large coversand ridge Maldegem-Stekene blocked the northwards outlet of the Valley and forced the River Schelde to bend eastward at Ghent and flow through the Land van Waas, over Antwerp, into the Oosterschelde (De Moor & Heyse, 1994). Moreover, the River Schelde transformed from a braided river system into a meandering one (Kiden, 1991). From the Late Glacial until today the general landscape remained more or less the same and was only slightly modified by aeolian processes, river incision and deposition. Nevertheless, these minor alterations made a further landscape division possible. One of these landscape units is the Moervaartdepression (De Moor, 1995).

Fig. 3: Evolution of the drainage system in the Flemish Valley since the last ice age (source: Van Zeir, 2009); 1: early Pleniglacial drainage pattern, 2: late Pleniglacial drainage, 3: Late Glacial and Holocene drainage pattern, with indication of the present-day rivers, 4: boundaries of the Flemish Valley according to De Moor (1963), 5: coversand ridge Maldegem-Stekene.

2. 4 The Moervaartdepression: geomorphological and hydrological evolution

The evolution of the depression is mainly based on Heynderickx (1982) and Crombé & Verbruggen (2002). However, new insights gained by the GOA project might alter the history as presented here (Fig. 4, A to H).

The late Pleniglacial landscape was dominated by a braided river system, as we already mentioned (Fig. 4A). In the following phase, the beginning of the Late Glacial (Fig. 4B), drainage of the Moervaartdepression happened in SW-NE direction (via Terwest) and gave rise to a series of relatively parallel SW-NE orientated ridges and plains in the western part of the depression. All major rivers and streams in the Flemish Valley had this drainage direction at the time, because the Upper-Schelde did not yet exist (Broothaers, 1995). Water came from the (south)west and (south)east in the early phase and changes into a simple (south)western inlet in a later phase.

The drainage of the Moervaartdepression completely changed when the large W-E directed coversand ridge Maldegem-Stekene was formed due to intensive aeolian sand blowing from what is now Zeeland and the North Sea. Water accumulation at the southern edge of the coversand ridge caused the erosion of broad shallow depressions. The former northwards drainage via Terwest evolved into an eastwards drainage system, which caused the expansion of the depression to the east and the creation of newer, smaller breakthrough valleys (Fig. 4C). As the coversand ridge developed and the breakthrough valleys disappeared the whole depression got dammed up. Fluvial supply and a rising groundwater table flooded the earlier created depressions and gave rise to a girdle of extended, but shallow lakes south of the coversand ridge. The remnants of the largest lake are nowadays known as the Moervaartdepression. Within the lakes sedimentation took place, creating marl deposits in the central and eastern part of the depression. These deposits are responsible for the lowland character of the area and are covered by younger sediments. Together with the infilling of the lake, the coversand ridge evolves into its current form (Fig. 4D).

At the beginning of the Holocene a lowering of the groundwater table caused the gradual disappearance of the lakes. Simultaneously, a gully system incised the Late Glacial and Pleniglacial sediments. Four main gullies can be observed. Amongst these are the early Holocene courses of the Moervaart and Zuidlede River which are located respectively north and south of the Moervaartdepression. The third gully sprung in the southwest, ran through the centre of the depression and joined the Moervaart southwest of Terwest. The fourth and central gully existed before marl sediments were deposited. Originally the drainage of the fourth gully happened in northwards direction but this changed when the coversand ridge Maldegem-Stekene was formed. From then on the fourth gully became connected to the Moervaart and Zuidlede gullies which drainage happened southwards via the Durme Valley (Fig. 4E). Eventually the central gullies filled up whereas the Moervaart and Zuidlede gullies remained present. The earlier mentioned lowering of the groundwater table turned the depression into a swamp which leads to peat formation, especially in the western part of the depression; the central part lost its function and the eastern part gained importance (Fig. 4F).

Fig. 4: Geomorphological and hydrological evolution of the Moervaartdepression (source: Heynderickx, 1982).

The last phase in the Moervaartdepression development was characterised by clay deposition. In the western part of the depression floods lead to the deposition of overbank alluvial clay along the Moervaart and Zuidlede River. In the eastern part the clay forms a mixture of fluvial clay and perimarine clay due from tidal activity in the Moervaart and Durme (Fig. 4G). Anthropogenic damming of the Durme River at Lokeren in 1955 changed the direction of the current in the area (Fig. 4H). Because of this, the Moervaart nowadays debouches in the canal Gent-Terneuzen (Spiers, 1986).

2.5 Present-day hydrography

The present-day hydrography is dominated by the Moervaart and Zuidlede River and numerous brooks, ditches and canals. The Moervaart and Zuidlede are respectively situated in the north and in the south of the depression. Despite the extensive drainage of the depression by these rivers and by the canal Gent-Terneuzen, the region still has a marshy character (Heynderickx, 1982).

The Moervaart, also known as the Noordlede, has been canalized and the latest excavations for the canalisation date from 1538. Two arms of the original Moervaart are still present between the canal Gent-Terneuzen and the actual Moervaart. Originally, the Moervaart flowed from west to east and discharged into the Durme River. Due to damming of the Durme at Lokeren in 1955 the water flows now from east to west and discharges into the canal Gent-Terneuzen (Heynderickx, 1982).

The Zuidlede more or less resembles the Moervaart. As the Moervaart, the Zuidlede is a W-E directed river, which was canalized in the Middle Ages and originally flowed from west to east. The damming of the Durme reversed the direction of the current. The Zuidlede splits from the Moervaart at Mendonk, bends south-east at Etbos and joins the Durme. The part after the bend at Etbos is almost certainly a canal (Heynderickx, 1982). The canalization along with other forms of human interference created several artificial river banks within the catchment of the Moervaart and Zuidlede River; these are detrimental for some freshwater molluscs.

2.6 Human occupation history

The earliest occupation of the Moervaartdepression dates back to the Final Palaeolithic. The depression and its surrounding area appeared to be attractive places at the time. Numerous traces of human occupation were found in and around the depression. The archaeological sites can be grouped into three clusters: one in the north-western part of the Moervaartdepression, a second in the north-east and a third one along the Durme River. The southern part of the Moervaartdepression, along the Zuidlede River, was inhabited to a lesser extent (Van Vlaenderen et al., 2006; Jongepier, 2009).

The different clusters were not all occupied at the same time (Fig. 5). During the Final Palaeolithic, human settlements were located in the western and eastern part of the Moervaartdepression, each time on slightly elevated terrain. The eastern sites are located at the edge of the coversand ridge of

Maldegem-Stekene. The western sites are mainly situated on a smaller sand ridge within the depression. In the next phase, the early Mesolithic, the occupation is extended towards the River Durme. At this time, some Final Palaeolithic sites were re-used. Such re-occupation makes analysis difficult and hazardous (Crombé & Verbruggen, 2002). During the late Mesolithic, the occupation pattern changes. There is an overall decline in the number of settlements and a shift to the north-western part of the Moervaart-depression, in the proximity of the Langlede River. This shift can be explained in various ways. The most likely explanation is a population decline, but a change in exploitation of the region is also possible. Finally, during the Neolithic, occupation is situated in the far west of the depression and along the Durme and Langlede River. The northern and northeastern parts are no longer attractive to prehistoric man (Van Vlaenderen et al., 2006; Jongepier, 2009).

Fig. 5: Distribution of Final Palaeolithic (upper left), Mesolithic (right) and Neolithic (lower left) sites within the Moervaart and Waasland regions (source: Jeroen De Reu, unpublished).

Molluscan ecology 3

Ecological factors are diverse and generally grouped as abiotic and biotic ones. The following overview is mainly based on Gittenberger & Janssen (2004) and Økland (1990). The taxonomical description of the recovered species at the end of this chapter is mainly based on Gittenberger & Janssen (2004) and Adam (1960). The taxonomic nomenclature follows Anderson (2005).

3.1 Abiotic factors

Abiotic factors are external environmental factors which have no biological origin whatsoever. Possibly the most important one of these factors is the amount of calcium in the water. Mollusc shells are composed of calcium carbonate (CaCO3) and snails are thus strongly relying on this component. Water with high calcium content (“hard water”) is certainly adventitious for mollusc communities. Other abiotic factors include the presence of an appropriate substrate (plants, rocks, sand etc.), the intensity of the water current, temperature and water quality.

3.1.1 Oxygen

The amount of oxygen dissolved in water is an important limiting factor not only for molluscs but for most aquatic organisms. Several factors determine the degree of oxygen dissolved in water. Temperature, water current, the amount of aquatic vegetation and decomposition of organic matter are some examples. However not all snails have the same oxygen requirements. Snails with gills are more sensitive to oxygen depletion than pulmonates which breathe at the water surface.

Different parameters have different effects on the water oxygen content. The amount of oxygen decreases with increasing temperature, but oxygen depletion can also take place at low temperatures. Winter depletion of oxygen is well‐known in small water bodies with high organic matter input as well as in ice covered lakes. Water currents have the opposite effect: currents and waves mix oxygen in water. Aquatic vegetation, both higher plants and duckweed, also increases the amount of oxygen during the day. Photosynthesis converts CO2 into O2, but at night the process reverses. This can cause drastic oxygen depletion at night. Silt‐rich riverbeds are also prone to oxygen reduction, certainly at high temperature. Snails tend to stay at the surface where the oxygen concentration is higher due to exchange of gasses between the atmosphere and the water. Exceptions to this rule are the large benthic freshwater mussels which have their own strategy. They provide themselves with new, oxygen‐rich water by means of self‐induced water currents.

3.1.2 pH and lime content

The hydrogen‐ion concentration (pH) and lime content of water bodies are strongly linked and are of great importance to organisms with a calcareous skeleton such as molluscs. Lime for shell formation can be obtained both from water as from plant digestion, since calcium is a nutrient of higher plants. If the lime concentration becomes too low, molluscs digest their own oldest whorls or start nibbling at each other’s shells. Molluscs can divided be into calciphile species, which require a high level of environmental calcium, and non‐calciphile species, i.e., species that are able to exploit a wide range of environmental calcium levels (Briers, 2003). The species recovered in our study are equally distributed across both groups.

In calcareous habitats, pH values will be quite neutral, around 7. These near‐neutral waters often contain the maximum benthic invertebrate diversity. Lime poor environments have a decreased buffering capacity and are therefore vulnerable to acidification through environmental pollution (acid rains). When pH reaches values below 5.5 more and more molluscan species disappear. Acidification not only causes lime shortage and shell corrosion, but also induces food shortage and organic matter and heavy metal accumulation, factors which are all detrimental for snail survival. Extremely high calcium carbonate concentrations are also detrimental for lacustrine productivity.

Phosphate and other essential nutrients absorb on CaCO3 particles and are thereby eliminated from nutrient cycling.

3.1.3 Salinity

Salinity is defined as the salt content of a body of water or the amount of dissolved salt ions in that water. Calcium (3.1.2) makes up a major part of the total salinity in most freshwaters. Salt content can be calculated by combining temperature and conductivity (EC20) measurements. In freshwater there is no fixed salt composition as opposed to seawater. Molluscs are divided into three large categories according to salinity. Freshwater molluscs and marine molluscs are respectively adapted to low and high salt contents. Brackish water molluscs resist fluctuating degrees of salt, ranging between seawater and freshwater. These are often estimates because for most species data are only available from certain locations or populations.

3.1.4 Currents and wave exposure

The environmental significance of currents and waves in relation to oxygen content was already mentioned (3.1.1), but their ecological importance does not end there. Wave action also determines the nature of the substratum. Habitats influenced by strong wave action are often rocky and stony, whereas more sheltered shores are characterised by finer substrata and dense vegetation, and have generally a more diverse malacofauna. Currents also play an important role in the mixing and renewal of water and the dispersal of molluscs. Strong currents drag molluscs downstream with them. For some this helps larval dispersal, for others it can lead to harmful displacement. Some bivalves adapt to strong currents by means of byssus threads or larger feet for better attachment to or in the substratum. Other molluscs make heavier shells.

3.1.5 Temperature

Temperature is an important abiotic factor. Not only does it influence the water oxygen content, it also determines metabolic activity as molluscs are exothermic animals that do not produce their own body heat. As the water temperature rises, molluscs become more active and grow faster. In spring, molluscs start to disperse and reproduce and this activity continues throughout summer. In autumn, mollusc activity declines and the animals move to the river bed, where they reside in winter and hibernate. Water temperature can be modified industrially. For example, cooling water discharge of power plants causes local unnaturally warm conditions throughout the year. Introduced frost sensitive species survive winter at these specific locations, whereas some native species cannot tolerate such conditions. Water temperature is also highly correlated with the amount of aquatic macrovegetation. Water basins with rich aquatic vegetation generally have a higher mean temperature than those with poor macrovegetation.

3.1.6 Water depth

Water depth is a less important abiotic factor, but its close linkage with a number of other abiotic deterministic factors increases its significance. Temperature, oxygen level, current, substratum and aquatic vegetation are some of the parameters linked with water depth.

In summer, solar radiation can divide the water body into two zone. The upper zone is penetrated by light and contains all kinds of aquatic plants and primary producers that rely on light for photosynthesis; the lower zone is not penetrated by light. The boundary between these zones lies at 6 to 10 m depth, according to the clearness of the water. In stagnant water basins, solar radiation can heat the upper layer without affecting the lower layer which has a constant temperature of approximately 4°C. In waters moved by wind, currents or waves, mixing of the water body creates more even temperatures. The lower zone of a lake generally consist of cooler and heavier water than the upper water mass causing it to stay in place at the bottom of the water basin creating a fairly stable environment. In this environment decomposition takes place. Organic matter formed in the upper layer sinks to the bottom of the water basin and its decomposition uses the available oxygen in the lower region causing oxygen poor conditions, sometimes even anoxia. The boundary between the upper and lower zones is thus not only a temperature but also an oxygen border.

3.1.7 Drought

Periodic drying of water bodies can have drastic effects on mollusc communities. Some species can resist a certain period of drought, others cannot. Small closed systems (ponds, ditches) and river banks are especially prone to desiccation. The risk of drying out has led some species to avoid shallow waters. In general, the number of species increases with increasing depth in shallow waters. Species which are specifically adapted to periodic drought are Galba truncatula (an amphibic species), Pisidium casertanum, Pisidium obtusale and Valvata cristata. Large freshwater mussels (e.g. Anodonta cygnea) on the contrary are very sensitive to dry conditions and are thus lacking from shallow water basins.

3.1.8 Pollution

The effect of pollution on the survival of freshwater molluscs is not well known. For several species a gradual decline has been observed in the last decades. However this supposed decline can be an observational artefact due to a lack of data on the present day situation caused by the general disinterest in our malacofauna. The relation between pollution and the presence of freshwater mussels has perhaps been documented the most. Continuous pollution since the 19th century onwards has banned large bivalves from most of the Scheldt River. Presently they can be found only in old abandoned meanders. In the River Maas which suffered less pollution, the same species can still be found in different locations (Prof. Dirk Van Damme, pers. comm. 2010). In the Netherlands, the same pattern was observed in the heavily polluted River Rhine during the sixties, but measures towards water quality amelioration have led to partial recovery of the malacofauna. Myxas glutinosa is also suffering from increasing habitat deterioration and destruction. It is especially associated with dense Stratiotes aloides vegetations. The continuing deterioration of these characteristic habitats is probably one of the causes for the observed decline in M. glutinosa. Generally speaking, studies by Wallbrink (1992, 1993) from the early nineties show that freshwater molluscs can be used as indicator species of the water quality. The disappearance of species occurs quickly. Reintroduction on the other hand takes longer and requires that species can travel from one location to another. For species with restricted dispersal capacities this might be an important limitation in the highly discontinuous landscapes, like in Belgium.

3.2 Biotic factors

Biotic factors relate to living creatures and their activities. They can influence both conspecifics and/or individuals of other species. In general, not much is known about the lives of molluscs and certainly not about freshwater molluscs. The lack of economical importance of snails has probably caused this general disinterest.

3.2.1 Food

Freshwater molluscs have diverse feeding habits. Most of them are herbivorous and feed on primary producers, like phytoplankton, algae and higher plants. Others eat detritus or scavenge. Physa fontinalis, Radix auricularia and Radix balthica eat dead snails, worms, mosquitoes and flies. Physa acuta occasionally feeds on dead frogs and fishes. Some snails are even carnivorous, such as Lymnaea stagnalis feeding on dead as well as living snails and dead flies and mosquitoes. Much scavengers or carnivorous snails grow faster when feeding on animal rather than on plant food. As opposed to most insects, molluscs seldom show specific feeding patterns; at most they show certain preferences. The majority of snails are generalists. This gives them the competitive advantage of feeding on what is available at the time. For some, the varied diet is a necessity, for other it’s merely a way of competing for food.

3.2.2 Enemies

Molluscan enemies can be divided into two categories: predators and parasites. The first category presents probably the major threat as snails and bivalves are food for a wide variety of animals including rats, birds, fishes, amphibians. According to some authors (e.g. Brönmark, 1988), vertebrate predation is a major structuring force of benthic gastropod communities. Predation by fish and crayfish is typical of large water basins, whereas predation by birds happens mostly in small water bodies. Next to vertebrates, invertebrates such as leeches, insects and especially insect larvae also feed on molluscs. Examples are dragonfly larvae, beetle larvae and larvae of various fly species, especially the mollusc killing Sciomyzidae. May flies use mollusc shells, even of living snails, to build their tubes. Snail parasites are mostly unicellular organisms, parasitic mites and trematodes which use snails as intermediate hosts in their life cycle. For instance, Galba truncatula is the intermediate host of the liver fluke (Fasciola hepatica) that infects various mammals, including humans.

Snails acquire adaptations to counter the foregoing threats. Many freshwater molluscs have inconspicuous colours ranging from yellow to grey‐brown shells. Others, especially Lymnaeidae, have thin shells through which the spotted mantel can be seen. Inconspicuous colours and spotted appearances are possibly a form of camouflage to protect the bearers against predators.

3.2.3 Competition

Competition between individuals occurs when resources (food, space) are limited. As already said, two types of competition can be distinguished: interspecific competition, that is, competition between individuals of different taxa and intraspecific competition or competition between individuals of the same species. Both interactions influence the well‐being of snail populations. Intraspecific competition for food can cause reduced growth, so that the population is composed of only small individuals; when competition for food diminishes this effect is reversed. The influence of interspecific competition is probably largest in small water basins where different species directly compete for the same resources. According to Økland (1990), but unexplained, it is possible that small snails are more affected by competition than large snails. This hypothesis of superiority of large species could possibly explain why Gyraulus crista, the smallest species, is most abundant when it occurs alone.

Competition can also be separated into two classes of mechanisms. Exploitative competition means that individuals, when using resources, rob others of possible benefits which could be gained from those resources (for example food). This is an indirect form of competition. Interference competition is a direct form which tends to the harm other molluscs by fighting, producing toxins etc.

3.2.4 Locomotion

Snails in general are not great travellers. They are slow and much dependent on the right environmental conditions for locomotion. Freshwater snails are less limited because they live in aquatic environments. They are mainly dependent on the right water temperature for locomotion, but great differences in locomotion capacities exist between species. Physidae are amongst the

18 fastest‐moving snails. The exotic, invasive species Physa acuta can even move upstream (de Kock & Wolmarans, 2007). Bivalves move the least of all, as they spend most of their live half buried in the substrate. Sphaeriidae are an exception: they possess a sticky foot with which they attach themselves to the substrate and crawl over it.

3.2.5 Dispersal mechanisms

Freshwater molluscs can be found in much isolated water bodies. The only way they can colonise such locations is through dispersion over land or through air. Passive dispersal over land is possible, for example through accidental attachment to animal fur. Dispersal through air is more likely and has been observed repeatedly; birds play an important role in this type of dispersion. Transport can happen in the gastrointestinal tract of birds or through attachment to paws or feathers. Bivalves and snails with an operculum stand a greater chance than pulmonates of reaching an isolated water basin alive, as they can close their shell. Parthenogenic species, that is, species that reproduce by means of unfertilised eggs and hermaphrodite species that can self‐fertilise have in theory the best chances to found new populations, as they need only one individual to colonise a new location. The colonisation and establishment of molluscan communities in isolated waters takes several years. Therefore, older lakes generally have richer communities than younger lakes.

Next to the former natural way of dispersal, man‐induced introduction of molluscs to new habitats has occurred in various ways, often unintentionally as a result of discharge of ballast water, transport attached to a surfboard or fishing gear etc. Artificial environmental modification form another important cause of molluscan community changes. For example, the building of dams causes former estuarine environments to become more and more freshwater dominated.

3.2.6 Reproduction

Freshwater molluscs have diverse reproduction strategies. Prosobranchia have male and female individuals and are thus of separate gender. The only representatives of this reproduction strategy in our study are Bithynia leachii and Bithynia tentaculata. The Valvatidae (Heterobranchia) and Pulmonata are hermaphrodites. Usually unilateral fertilisation takes place, that is, one snail acts as male, the other as female. Bilateral fertilisation also exists in specific groups of Pulmonata, for example the Planorbidae. After a first unilateral insemination the reproductive roles are switched for the second fertilization. Various species of Planorbidae also self‐fertilise.

Like the Prosobranchia bivalves are often separated by gender. Dreissenidae has a free swimming larval stadium. Sphaeriidae are hermaphrodite and exhibit brood care. The inner gills grow together into a kind of pouch (marsupium) which contain the eggs. Bivalves continuously refresh the water inside their valves by means of an in and out flowing current. Sperm cells enter the marsupium along with this water current and fertilise the eggs. In the pouch the fertilised eggs develop into small mussels, after which they leave their mother. Unionidae are hermaphrodite and have a parasitic larval stage. Glochidia attach themselves to fish gills by means of small hooks, live on fish blood for nine to ten month and develop into small mussels, after which they leave their host.

3.3 Water types

Each water type is characterised by a certain combination of water quality, physical‐hydrological and geomorphological factors. In general, a coarse division of water types suffices for molluscs, because they are less sensitive to environmental differences than other macro‐invertebrates. Intermediate situations and human influences (canalisation, damming, eutrophication, acidification etc.) often alter the original molluscan community.

Økland (1990) discusses the snail fauna of various surface water types based on his own observations. According to him, puddles and rapid‐flowing rivers have low species diversity. For puddles, this is due to the fact that they represent environmental unstable systems: they are prone to desiccation in summer and freezing to the bottom in winter because of their small volume. Besides, they contain only a small number of niches. The low diversity in rapid‐flowing rivers is probably linked to the effects of flowing water. Maximum species diversity was found in lakes, because lakes are characterised by maximum environmental stability, maximum number of niches, and maximum probabilities of immigration. There is also no disturbance by rapid flowing water. In between the foregoing extremes, mires, ditches, ponds and slow‐flowing rivers are characterised by an intermediate number of species. Mires, ditches and ponds have intermediate water volume, water depth and surface area, but also an intermediate environmental stability, number of niches etc. These factors combined with general higher water hardness make mires, ditches and ponds attractive to snails. The intermediate position of slow‐flowing rivers opposed to lakes is not necessarily correct in every country. In Norway, the country examined by Økland, slow‐flowing rivers are less adventitious for snail because they become rapid‐flowing in spring when filled with melt water. In Britain, the opposite situation is observed: slow‐flowing rivers have the highest diversity. With respect to lakes, we can thus conclude that topology and climate also play an important role in determining the malacofauna of different water types.

Molluscan communities are also determined by the existing fish population. Predation by molluscivorous fishes can lead to less diverse communities and especially snails with thin shells are vulnerable to predation. If the food chain is extended and piscivorous fishes are added, the equilibrium shifts: the more predators, the less molluscivorous fishes but the more snails. However, the total molluscan biomass in the latter situation is still lower than in waters without fish. The presence and dominance of eight molluscs was examined in three types of food chains by Brönmark & Weisner (1996). In the complete absence of fishes, Lymnaea stagnalis dominated, followed by Bathyomphalus contortus, Anisus vortex, Radix balthica, Gyraulus crista, Physa fontinalis, Gyraulus albus and Bithynia tentaculata. If only molluscivore fishes were present, the distribution shifted to the following sequence: Bithynia tentaculata, Lymnaea stagnalis, Radix balthica, Gyraulus albus, Gyraulus crista, Anisus vortex and Bathyomphalus contortus. Physa fontinalis was practically lacking in this situation. In the presence of both molluscivorous and piscivorous fishes Anisus vortex and Gyraulus albus dominated, followed by Bithynia tentaculata, Lymnaea stagnalis, Physa fontinalis, Radix balthica, Gyraulus crista and Bathyomphalus contortus. The equilibria also influence the amount of primary producers in the water. The more molluscs, the more grazing, the less primary producers and the cleaner the water.

3.4 Taxonomic description of the recovered species

Phylum MOLLUSCA Class GASTROPODA Subclass PROSOBRANCHIA Superorder CAENOGASTROPODA Order NEOTAENIOGLOSSA Superfamily RISSOIDEA Gray, 1847 Family BITHYNIDAE Troschel, 1857

Genus Bithynia Leach, 1818 Subgenus Bithynia s.str.

Bithynia leachii Sheppard, 1823 Description: Shell cone‐shaped, approximately 1½x higher then broad,. Maximum 5 convex whorls. Mouth opening practically round till slightly oval. Parietal side of the mouth edge makes contact with the former whorl. Umbilicus narrow, but visible. Operculum calcified, with a central nucleus and concentric rings. Dimensions: Height until 7.0 mm, width until 4.5 mm. Ecology: B. leachii prefers stagnant fresh water with rich plant growth. Maximum salinity of 5‐6‰. Fossil occurrence: Interglacials and interstadials. Rare, but more general during the Holocene. Recent distribution: North America, Europa and eastwards until East Siberia. Remark: Modern records only.

Bithynia tentaculata Linnaeus, 1758 (Plate 1 I & J) Description: Shell cone‐shaped, approximately 1½x higher then broad. Maximum 6 moderately convex whorls. Mouth opening egg‐shaped, distinctly pointed at the top. Parietal side of the mouth edge makes broad contact with the former whorl. The umbilicus is normally closed off by the mouth edge. Operculum calcified, with a more or less central nucleus, surrounded by concentric rings and obviously pointed at the top. Dimensions: Height until 12.0 mm, width until 7.0 mm. Ecology: Various stagnant or (slow) flowing fresh till brackish waters, with a more or less rich plant growth. Minimum pH of 5.0, maximum salinity of 12‰. Frost‐sensitive. Fossil occurrence: Common species, known from interglacials and interstadials. Often only opercula are found, because they are composed of a stronger form of CaCO3, namely calcite. Recent distribution: Northwest Africa, large parts of Europe and eastwards until West Siberia. Introduced in America.

Subclass HETEROBRANCHIA Superorder HETEROSTROPHA Order ECTOBRANCHIA Superfamily VALVATOIDEA Gray, 1840 Family VALVATIDAE Gray, 1840

Genus Valvata Müller, 1774 Subgenus Cincinna Hübner, 1810

Valvata piscinalis Müller,1774 (Plate 1F) Description: Shell convex cone‐shaped and approximately as broad as high. Mouth opening more or less round. Broad umbilicus. Operculum with 6 to 7 spiral coils. Dimensions: Height and width until 6.0 mm. Ecology: Stagnant to slow flowing water with rich plant growth. Sometimes in very high densities: thousands of individuals can be found per m2. Minimum pH of 5.4, maximum salinity of 5.2‰. Fossil occurrence: Present throughout the whole of the Quaternary, especially during interglacials and interstadials. Recent distribution: Common in many places in Europe and West Asia. Introduced in North America at the start of the 20th century.

Subgenus Valvata s.str.

Valvata cristata Müller, 1774 (Plate 1Q) Description: Shell normally disk‐shaped (i.e. all whorl lie in one plain). Mouth opening round. Umbilicus covering approximately ⅓ of the total width of the shell. Operculum with 6 to 7 spiral coils. Dimensions: Height until 1.5 mm, width until 3.5 mm. Ecology: Preference for calm freshwater. Minimum pH of 5.0, maximum salinity of 5.2‰. Resistant to periodic drying of their habitat. Fossil occurrence: Known from most interglacials and some interstadials and usually lacking from colder periods. Recent distribution: Widespread throughout Europe with exception of the areas around the Mediterranean Sea.

Order PULMONATA Suborder BASOMMATOPHORA Infraorder HYGROPHILA Superfamily ACROLOXIDEA Thiele, 1931 Family ACROLOXIDAE Thiele, 1931

Genus Acroloxus Beck, 1838

Acroloxus lacustris Linnaeus, 1758 (Plate 1D) Description: Shell long‐drawn cap‐shaped, almost twice as long as broad. Parallel sides. The obvious pointed tip is located to the left of the medial line, in the caudal part of the shell. Dimensions: Length until 8.5 mm, width until 4.0 mm, height until 2.0 mm. The length/width ratio varies between 1.4 and 2.2. Ecology: Typical of stagnant, slightly larger water basins like likes and canals with a preference for deeper waters (up till more than 150 cm). Also found in large rivers, but not in places with strong currents. Some wave exposure can be tolerated. Minimum pH of 5.4, maximum salinity of 3‰. Obvious preference for warmer water since it can even be found at water temperatures of 30°C. Fossil occurrence: Interglacials.

Recent distribution: Throughout the whole of Europe except for the regions north of South Scandinavia and in certain parts of Asia.

Family CARYCHIIDAE Jeffreys, 1830 Genus Carychium Müller, 1773

Carychium minimum Müller, 1774 (Plate 2N) Description: Shell compact spindle like, relatively wide. Maximum 4½ moderate convex whorls. Mouth edge thickened and turned. Palatal mouth edge with a knob in the middle; one columellar tooth and one parietal tooth. Opening up the shell with a fine needle in order to see the internal course of the parietal blade can aid determination. Dimensions: Height until 1.9 mm, width until 0.9 mm. Ecology: Moist habitats, such as swamps or very moist forests. Recent distribution: Europe and Siberia. Remark: Only found as subfossil. Superfamily LYMNAEOIDAE Lamarck, 1812 Family LYMNAEIDAE Lamarck, 1812

Genus Galba Schrank, 1803 Subgenus Galba s.str.

Galba truncatula Müller, 1774 (Plate 1G) Description: Shell cone‐shaped to high cone‐shaped, slightly more than twice as high as broad. Maximum 5½ convex to very convex whorls, separated by a deep suture; upper whorls flattened and stepwise deposited. The last whorl covers approximately 70% of the total shell height. Umbilicus visible as a narrow slit. Dimensions: Height until 9 mm (exceptionally until 15 mm), width until 5 mm. Ecology: Can live both in water and on land, but always in moist conditions. If found in fresh or moderate brackish water never deeply beneath the water surface. Resistant to drought in all life stages (eggs, juveniles and adults) and also quite resistant to frost. Temperatures above 20°C are suboptimal and above 25°C harmful. Fossil occurrence: Known from Quaternary interglacials and interstadials, but also from some colder periods like the Weichselian and Holstein. Recent distribution: Throughout Europe, (North) Africa, West and North Asia and Alaska. Remark: Only found as subfossil.

Genus Lymnaea Lamarck, 1799

Lymnaea stagnalis Linnaeus, 1758 (Plate 1A) Description: Shell approximately twice as high as wide. Maximum 8 whorls, separated by a clear, but shallow suture. Flattened top whorls which gradually increase in size; strongly widened and enlarged last whorl that covers almost ¾ of the total shell height. Mouth opening broadly oval, round at the bottom, pointed at the top, covering almost half of the total shell height. Closed umbilicus. Dimensions: Height until 70 mm, width until 33 mm.

Ecology: Predominantly present in stagnant fresh to weakly brackish well‐vegetated waters. Maximum salinity of 7‰. Frost‐sensitive. Fossil occurrence: Interstadials and interglacials. Recent distribution: Holarctic. Throughout Europe with exception of the areas around the Mediterranean Sea.

Genus Myxas Sowerby, 1822 Subgenus Myxas s.str.

Myxas glutinosa Müller, 1774 (Plate 1H) Synonym: Amhipeplea glutinosa. Description: Shell remarkably thin and very fragile, practically spherical, only slightly higher than wide, with a maximum of 3½ whorls which increase in size regularly and fast. Apex blunt, hardly rising above the strongly swollen last whorl, which covers practically all former whorls. Mouth opening broadly oval, slightly pointed at the top, covering almost the whole of the shell height. Mouth edge sharp and extremely delicate. Umbilicus absent. Dimensions: Height until 15 mm, width until 13 mm. Ecology: Prefers calm, stagnant, well‐vegetated waters, especially thick Stratiotes aloides vegetations. Present to lesser extent in slow flowing water. Can resist short periods of freezing (de Bruyne et al., 2008). Maximum salinity of 3‰. Fossil occurrence: Known from Holocene gyttja deposits in the Netherlands, together with seed of Stratiotes aloides. Recent distribution: Lowlands, in large parts of Europe, eastwards until West Siberia. Remarks: Only found as subfossil. Recently steeply declining due to pollution and habitat destruction.

Genus Radix Montfort, 1810 Subgenus Radix s.str.

Radix auricularia Linnaeus, 1758 (Plate 1C) Description: Shell approximately as wide as high, or slightly higher. Maximum five whorls which increase in size gradually at first and then faster. Last whorl big and swollen, covering almost half of the total shell height. Umbilicus slightly covered by the mouthedge, but even so quite obvious. Dimensions: Height until 35 mm, width until 30 mm. Ecology: Stagnant, well‐vegetated, somewhat larger waters like ditches, canals, old river arms and puddles. Maximum salinity of 6‰. Fossil occurrence: Interglacials. Recent distribution: Europe with exception of Sicily and the Peloponnese and North Asia. Remark: Only found as subfossil.

Radix balthica Linnaeus 1758 (Plate 1B) Synonym: Radix ovata Draparnaud, 1805 Description: Shell highly variable, but generally clearly higher than wide. Last whorl large and swollen. Mouth opening broad and oval, slightly pointed at the top. Umbilicus largely covered by the callus.

Dimensions: Height until 31 mm, width until 20 mm. Ecology: On plants, rocks and mud in highly variable fresh and slightly brackish waters, like ditches, rivers and lakes. Moderately tolerant to pollution. Maximum salinity of 14‰. Fossil occurrence: Known from the whole of the Quaternary, both interglacials and interstadials. Recent distribution: Europe and North and West Asia.

Genus Stagnicola Jeffreys, 1830

Lymnaea palustris Müller, 1774 Description: Shell high and cone‐shaped, approximately twice as high as wide. Maximum six moderately convex whorls; last whorl slightly swollen. Deeply incised suture. The height of the mouth opening is mostly slightly less than half of the total shell height. Umbilicus absent. Highly variable shell shape. Dimensions: Height until 20 mm, width until 8.5 mm. Ecology: Generally present in stagnant fresh to slight brackish well‐vegetated water. Sometimes in flowing water. Can survive outside the water for a limited period. Maximum salinity of 9.5‰. Fossil occurrence: Interglacials and interstadials. Recent distribution: North Africa, Europe and large parts of Asia. Remark: Modern records only.

Superfamily PHYSOIDEA Fitzinger,1833 Family PHYSIDAE Fitzinger, 1833

Genus Physa Draparnaud, 1801 Subgenus Physa s.str.

Physa fontinalis Linnaeus, 1758 (Plate 1E) Description: Shell left wound, egg‐shaped, approximately 1½ times as high as wide. Apex blunt and rounded. Maximum four whorls; last whorl large and swollen, covering 4/5 or more of the total shell height. Mouth opening high, broad at the bottom and narrow, pointed at the top. Mouth edge sharp. Umbilicus absent. Dimensions: Height until 15 mm, width until 9 mm. Ecology: Physa fontinalis is frequently found in stagnant fresh to slight brackish water with rich vegetation. It is much less abundant in flowing water. Maximum salinity of 6‰. Fossil occurrence: Interglacials and interstadials. Recent distribution: Almost throughout Europe and in large parts of North and Central Asia and in Canada.

Genus Physella Haldeman, 1843 Subgenus Costatella Dall, 1870

Physella acuta Draparnaud, 1805 Description: Shell left wound, approximately egg‐shaped, 1½ times as high as wide. Apex ending in a sharp point. Maximum six whorls; last whorl large and swollen, sometimes somewhat shouldered,

25 covering seventy percent of the total shell height. Mouth opening high, broad at the bottom and narrow, pointed at the top. Mouth edge sharp. Umbilicus absent. Dimensions: Height until 19 mm, width until 14 mm. Ecology: Mostly present in large water basins, like canals and puddles. Maximum salinity of 8‰. Physella acuta is a thermophilic species which flourishes even at water temperatures of 35°C. Temperature fluctuations of 6 to 30°C, or vice versa, do not harm them. This characteristic explains why they are often found at discharge points of industrial cooling water. Physella acuta cannot survive periodic drying of its habitat. Fossil occurrence: Unknown from Belgium as it was probably only introduced in 1869 (Adam, 1960). Recent distribution: Originally coming from southwest Europe and North Africa; elsewhere introduced. Remark: Modern records only, especially in polluted waters.

Superfamily PLANORBOIDEA Rafinesque, 1815 Family PLANORBIDAE Rafinesque, 1815 Subfamily PLANORBINAE Rafinesque, 1815 Tribus PLANORBEAE Rafinesque, 1815

Genus Anisus Studer, 1820 Subgenus Disculifer Boettger, 1944

Anisus vortex Linnaeus, 1758 (Plate 1L) Synonym: Planorbis vortex Description: Shell approximately seven times as wide as high. Upper side flat, underside somewhat sunken. Maximum seven whorls which gradually increase in size. Last whorl clearly keeled at the upper side. Mouth opening broader than high. Dimensions: Width until 13 mm, height until 1.8 mm. Ecology: A wide variety of stagnant and slow flowing, well‐vegetated waters. Maximum salinity of 8.4‰. Fossil occurrence: Interglacials and interstadials, but scarcely. Only frequently found in the Holocene. Recent distribution: Europe and West Asia. Remark: Found in both fossil sequences but only in very small numbers. More abundant in the recent sampling sites.

Genus Bathyomphalus Charpentier, 1837 Subgenus Bathyomphalus s.str.

Bathyomphalus contortus Linnaeus, 1758 (Plate 1O) Synonym: Planorbis contortus Description: Shell characteristic and thick; 2½ to 4 times as wide as high. The shell resembles an enrolled belt as its Dutch names, that is, Riempje indicates. Upper side obviously sunken, underside almost flat. Maximum eight whorls which gradually increase in size. Mouth opening clearly higher than wide. Mouth edge attached to the penultimate whorl very low at the underside and high at the upper side. Dimensions: Width until 7.0 mm, height until 2.0 mm.

Ecology: Various stagnant or slow flowing, well‐vegetated waters. Very susceptible to periodic drying of its habitat, but resistant to some degree of pollution. Minimum pH of 5.0, maximum salinity of 8.4‰. Fossil occurrence: Known from interglacials, but rare. Recent distribution: Large parts of Europe, with exception of the most southern regions, and further eastwards throughout Siberia until the basin of the Amoer. Remark: Scarce species, both fossil and recent.

Genus Gyraulus Charpentier, 1837 Subgenus Armiger Hartmann, 1843

Gyraulus crista Linnaeus, 1758 (Plate 1P) Synonyms: Planorbis crista, Armiger crista. Description: Shell approximately four times as wide as high. Upper side sunken, underside flat or slightly convex. Maximum three whorls which gradually increase in size. The last whorl displays a weak to strong rounded keel at the underside. The mouth edge is attached to the upper side and the periphery of the penultimate whorl. Dimensions: Width until 4.0 mm, height until 1,0 mm. Ecology: Gyraulus crista is present in stagnant to slow flowing, mostly well‐vegetated, fresh to slight brackish waters. Minimum pH of 5.0, maximum salinity of 5.2‰. The species does not survive periodic drying of its habitat. Fossil occurrence: General species known from most of the Quaternary, especially but not exclusively from interglacials. Recent distribution: Holarctic, Europe with exception of the high latitudes. Remark: All Gyraulus crista individuals, both fossil and recent, belonged to the forma cristata. This form displays approximately fifteen radial ribs on the surface of the last whorl which protrude the periphery. The ribs are equally spaced and are more obvious on the upper side than on the underside of the shell.

Subgenus Gyraulus s.str.

Gyraulus albus Müller, 1774 (Plate 1M) Synonym: Planorbis albus. Description: Shell three times as wide as high. Upper side sunken, underside flat or slightly sunken. Maximum 4½ whorls which increase in size gradually but rapidly. Last whorl evenly rounded, mostly without keel. The surface is covered with spiral lines and growth lines which creates a characteristic reticular pattern. Dimensions: Width until 9.0 mm, height until 2.0 mm. Ecology: Various types of stagnant or flowing freshwater with more or less aquatic vegetation. Minimum pH of 5.2, maximum salinity of 5.2‰. The species often appears as one of the first in newly created waters (pioneer species) and is mostly present in small numbers. Fossil occurrence: Scarce species known from most of the Quaternary, but almost exclusively from interglacials. Recent distribution: Holarctic, large parts of Eurasia and North America.

Genus Planorbis Müller, 1774 Subgenus Planorbis s.str.

Planorbis planorbis Linnaeus, 1758 (Plate 1K) Description: Shell approximately five times as wide as high. Upper side flat or slightly sunken, underside slightly sunken in the middle. Maximum six whorls which gradually increase in size. The whorls are flattened at the upper side and strongly bend at the underside. Last whorl with a sharp keel. Mouth opening approximately as wide as high. Dimensions: Width until approximately 20 mm, height until approximately 4 mm. Ecology: Particularly in stagnant well‐vegetated waters and on muddy soil. Sometimes present in large numbers. Planorbis planorbis can live in weak brackish water, tolerates some degree of pollution and survives temporary drying of its habitat. Minimum pH of 5.0, maximum salinity of 11‰. Fossil occurrence: General species, known from most of the Quaternary. Recent distribution: Europe, Africa north of the Sahara and West Asia.

Tribus SEGMENTINEAE Baker, 1945

Genus Hippeutis Charpentier, 1837 Subgenus Hippeutis s.str.

Hippeutis complanatus Linnaeus, 1758 (Plate 1N) Synonym: Planorbis complanatus Description: Shell approximately four times as wide as high. Upper side slightly convex and clearly sunken in the middle. Maximum four whorls which gradually increase in size and cover each other. Broad last whorl. Mouth opening deeply incised by the penultimate whorl. Dimensions: Width until 5.1 mm, height until 1.5 mm. Ecology: Stagnant and slow flowing, mostly well‐vegetated waters. Not in periodic drying or strong brackish water. Minimum pH of 6.1, maximum salinity of 2‰. Almost never in large numbers. Fossil occurrence: Interglacials but scarce. Recent distribution: Europe, Africa north of the Sahara and West Asia.

Subfamily HELISOMATINAE Baker, 1928

Genus Planorbarius Froriep, 1806 Subgenus Planorbarius s.str.

Planorbarius corneus Linnaeus, 1758 Synonym: Planorbis corneus. Description: Shell strong, big, approximately 2½ as wide as high. Upper side slightly sunken, underside deeply sunken. Maximum six convex whorls which increase in size gradually but rapidly. The whorls are separated from one another by a deep suture. Mouth opening approximately as wide as high and slightly incised by the penultimate whorl. Dimensions: Width until 35 mm, height until 15 mm.

Ecology: Present in stagnant and slow flowing well‐vegetated waters. Minimum pH of 6.0, maximum salinity of 3‰. Absent from very shallow and periodic drying waters. Fossil occurrence: Known from practically all interglacials of the Quaternary. Recent distribution: Europe and West Asia. Remark: Modern records only.

Family PUNCTIDAE Morse, 1864

Genus Punctum Morse, 1864 Subgenus Punctum Morse, 1864

Punctum pygmaeum Draparnaud, 1804 (Plate 2P) Description: Shell very small, disc‐shaped with a weakly elevated apex. Maximum 3½ moderately vaulted whorls, segregated by a deep suture. Umbilicus wide. Mouth opening rounded, mouth edge thin. Dimensions: Width until 1.5 mm. Ecology: Various more or less moist, shady places; especially abundant in the litter of deciduous forests; also in swamps. Recent distribution: Eurasia and North America. Remark: Only found as subfossil.

Family PUPILLIDAE Turton, 1831

Genus Pupilla Fleming, 1828 Subgenus Pupilla Fleming, 1828

Pupilla muscorum Linnaeus, 1758 (Plate 2L) Description: Shell rounded cylindrical. Maximum six or seven slight convex whorls; the last whorl is often slightly smaller than the penultimate whorl. Mouth opening with a small parietal protrusion. Mouth edge faintly turned and enlarged. Dimensions: Height until 4 mm, width until 1.7 mm. Ecology: Dry open lime rich habitats such as dry grasslands. Recent distribution: Eurasia and North America Remark: Only found as subfossil.

Family SUCCINEIDAE Beck, 1837

Genus Succinea Draparnaud, 1801

Succinea putris Linnaeus, 1758 (Plate 2J) Description: Shell with three whorl which rapidly increase in size. The last whorl covers approximately ⅔ of the total shell height. Whorls moderately convex, shallow suture. Mouth opening wide and regularly rounded. Shell fragile. Dimensions: Height until 17 mm, width until 10 mm.

Ecology: Mostly in close proximity of water: along rivers, lakes, ditches, in swamps or moist grasslands. Recent distribution: Europe and Siberia. Remark: Only found as subfossil.

Genus Succinella Mabille, 1870

Succinella oblonga Draparnaud, 1801 (Plate 2K) Synonym: Succinea oblonga Draparnaud, 1801 Description: Shell slender with approximately three slight convex whorls which increase in size gradually but rapidly. Mouth opening oval and covering approximately half of the total shell height. Dimensions: Height until 8 mm, width until 6 mm. Ecology: More or less moist slightly vegetated places; along rivers, in swamps; typical of desiccating bare mud plains. Generalist species. Recent distribution: Europe and West Asia. Remark: Only found as subfossil.

Family VALLONIIDAE Pilsbry, 1900

Genus Vallonia Risso, 1826

Vallonia pulchella Müller, 1774 (Plate 2O) Description: Shell disc‐shaped with a weakly elevated apex. Maximum 3¼ convex unshouldered whorls which increase in size regularly. Mouth edge strongly thickened and bent outwards. Dimensions: Width until 2.5 mm. Ecology: Open lime‐rich habitats: moist meadows, swamps, dunes. Sometimes present in dry grasslands, never in forests. Recent distribution: Eurasia and North America. Remark: Only found as subfossil.

Family VERTIGINIDAE Fitzinger, 1833

Genus Vertigo Müller, 1773 Subgenus Vertigo Müller, 1773

Vertigo sp. (Plate 2M) Description: Shell more or less ovoid with four slightly convex whorls, separated by a superficial suture. Dimensions: Height 2.2 mm, width 1.4 mm. Remark: Only found as subfossil. Identification to species level impossible since the last whorl had broken off. Possibly Vertigo antivertigo (Draparnaud, 1801) based on the habitat preferences of the other terrestrial species.

Class BIVALVIA Subclass HETERODONTA

Order VENEROIDA Superfamily DREISSENOIDEA Gray, 1840 Family DREISSENIDAE Gray, 1840

Genus Dreissena Van Beneden, 1835 Subgenus Dreissena s.str.

Dreissena polymorpha Pallas, 1771 Description: Shell long‐drawn, strong, irregularly triangular shell with an obvious umbo positioned at the front of the shell. Approximately twice as long as high. The upper side displays an obvious kink whereas the underside is straight or slightly concave. The shell is strongly keeled which gives it a triangular shape on cross section. Dimensions: Length until 40 mm, width until 20 mm, depth until 24 mm. Ecology: Present in fresh to weak brackish waters with a maximum salinity of 4.7‰. Higher salinities can be endured during short periods. The species displays a high tolerance towards various forms of pollution and is therefore used as an indicator species of pollution. Dreissena polymorpha prefers larger water basins and can live in both stagnant and flowing water. Short periods of drought can be overcome. Solid object serve as substrate, for example rocks, wood, iron and other shells. Owing to their mass occurrence they can be troublesome. Fossil occurrence: Unknown. Recent distribution: Europe (eastwards until the Ural Mountains) and North America. Remark: Modern records only.

Superfamily SPHAERIOIDEA Deshayes, 1854 Family SPHAERIIDAE Deshayes, 1854 Subfamily PISIDIINAE Gray, 1857

Genus Pisidium Pfeiffer, 1821

Pisidium amnicum Müller, 1774 (Plate 2A) Description: Shell oval, sometimes somewhat triangular, but always longer than high. Strong, usually with robust ribs (1 or 2 every 0.5 mm), which become weaker and less spaced towards the umbo. Cardinal teeth: c3 is strongly bent, c4 is short and stretched and lies obliquely behind c2. Dimensions: Length until 11 mm, height until 8.5 mm and width until 6.5. Ecology: Moving waters of rivers, brooks, canals, lakes and ponds and smaller water basins that are linked to the former. Maximum salinity of 0.5‰. Fossil occurrence: Known from Europe since the Early Pliocene. General in interglacials and interstadials throughout the Quaternary. Especially in fluvial deposits. Recent distribution: Throughout Europe, but more general north of the Pyrenees and Alps than around the Mediterranean Sea. Remark: Only found as subfossil.

Pisidium casertanum Poli, 1791 (Plate 2B) Synonyms: Pisidium cinereum Alder, 1838; Pisidium colbeaui Clessin, 1879.

Description: Shell oval to sub‐triangular. Umbo submedial. Sculpture composed of irregular striping, sometimes weak ribs. Cardinal teeth: c2 angularly bent, c4 short and straight, obliquely behind c2; c3 weakly bent, thickened at the end. Dimensions: Length until 5.5 mm, height until 4.6 mm, diameter until 3.4 mm. Ecology: Present in rivers, brooks, canals, ditches, lakes, ponds, swamps and period drying water. Minimum pH of 4.6. Fossil occurrence: Known from Europe since the Early Pliocene. Recent distribution: Cosmopolitan. Remark: Only found as subfossil.

Pisidium milium Held, 1836 (Plate 2C) Description: Shell trapezoid, shiny and often very swollen: the diameter of the shell is often larger than the height. Broad umbo, often sticking out far beyond the upper edge of the shell. Cardinal teeth short, thin and straight. Dimensions: Length 3.0 mm, height 2.4 mm, diameter 2.1 mm. Ecology: Present in a multitude of waters, like rivers, brooks, pools, pot‐holes, polders‐, sand‐, peat‐ ditches, etc. Maximum salinity of 0.5‰. Fossil occurrence: Known from the Early Pleistocene onwards. Recent distribution: Palearctic. To the north no further than southern Scandinavia, to the south only scarcely found around the Mediterranean Sea. Some relict populations in the Near East. Remark: Only found as subfossil.

Pisidium nitidum Jenyns, 1832 (Plate 2D) Description: Shell somewhat trapezoid, shiny. The umbo is surrounded by three, four of five ribs whereas the rest of the shell displays vague ornamentation. Cardinal teeth: short and straight. Dimensions: Length 3.5 mm, height 2.9 mm, diameter 2.3 mm. Ecology: A wide variety of waters. Similar biotope as Pisidium milium and Pisidium subtruncatum. Fossil occurrence: Known since the Late‐Plioceen. Recent distribution: Holarctic. In Europe from the Pole circle to North Africa. Remark: Only found as subfossil.

Pisidium obtusale Lamarck, 1818 (Plate 2E) Description: Shell oval with an approximately medial positioned, convex and broad umbo. Vague sculpture. Teeth: p3 has a lumpy thickening at the proximate side. Dimensions: Length 3.0 mm, height 2.6 mm, diameter Ecology: Typical of acidic, lime‐poor biotopes. Swamps, peat ditches, puddles and fens, always in stagnant waters. Pisidium obtusale can also be found outside the water in moist Sphagnum mosses. The species is cold resistant and freezes only at ‐11.1°C. Fossil occurrence: Known in Europe from the Late Pliocene onwards. Recent distribution: Holarctic. Common throughout Europe except in the Mediterranean region. Remark: Only found as subfossil.

Pisidium pulchellum Jenyns, 1832 (Plate 2F) Description: Shell obliquely oval, low umbo, clearly behind the middle of the shell length. Regularly ribbed, with a silky shine. Cardinal teeth: c2 and c4 straight and parallel.

Dimensions: Length 3.5 mm, height 3.1 mm, diameter 2.3 mm. Ecology: Present in small clear water basins, in the sandy‐muddy substrate of ditches and canals, brooks, depeated puddles, always in the company of other Sphaeridae and in small numbers. Fossil occurrence: Known in Europe from the Late Pleistocene onwards. Recent distribution: Palearctic. Remark: Only found as subfossil.

Pisidium subtruncatum Malm, 1855 (Plate 2G) Description: Shell obliquely oval, pointed at the front and somewhat flattened at the back. Oblique umbo, positioned far backwards. Cardinal teeth: c2 short and straight, c4 longer, stretched and parallel to c2. Dimensions: Length 3.5 mm, height 2.8 mm, diameter 2.2 mm. Ecology: Present in a wide variety of water basins, but not in Sphagnum mosses. Maximum salinity of 3‰. Fossil occurrence: Mainly known from the Early Pleistocene in Europe. Recent distribution: Holarctic, but in North America much less frequent than in Europe. Remark: Pisidium subtruncatum is the only Pisidium species which was present in both the fossil and recent malacofauna.

Pisidium tenuilineatum Stelfox, 1918 (Plate 2H) Description: Shell oval, clearly longer than high, umbo positioned behind the medial line. Closely and regularly ribbed. Dimensions: Length 1.9 mm, height 1.5 mm, diameter 1.2 mm. Ecology: Present in calm river bends, in clear brooks and in the littoral zone of lakes. Sensitive to water pollution. Fossil occurrence: Known in Europe from the Late Miocene onwards, in the Netherlands known from the Holocene. Recent distribution: Western palearctic. Absent in North America. Remark: Only found as subfossil.

Subfamily SPHAERIINAE Deshayes, 1854

Genus Musculium Link, 1807

Musculium lacustre Müller, 1774 Description: Shell variable in shape, oval to round, or rounded square to approximately triangular, higher behind the umbo than in front. Very thin and fragile. Characteristic small, oval, convex embryonic shell visible as a clearly marked cap on the umbo. Dimensions: Length 10.0 mm, height 8.0 mm, diameter 5.0 mm. Ecology: Stagnant waters, ditches, canals, depeated pools, sandpits, fens etc. Frequently present in periodic drying waters. Minimum pH of 6.1, maximum salinity of 3‰. Fossil occurrence: Known in Europe from the Late Pliocene onwards. Recent distribution: Holarctic. Remark: Modern records only.

Genus Sphaerium Scopoli, 1777 Subgenus Sphaerium s.str.

Sphaerium corneum Linnaeus, 1758 (Plate 2I) Description: Shell oval, rather convex; thin but strong; irregularly and weakly ribbed. Umbo broad and low, positioned roughly in the middle. Dimensions: Length 10.0 mm, height 8.5 mm, diameter 6.5 mm. Ecology: Various aquatic habitats, both flowing and stagnant, but not in temporary drying water basins. Sphaerium corneum can resist a high degree of organic pollution. Minimum pH of 5.4, maximum salinity of 5.2‰. Fossil occurrence: Interglacial and interstadials since the Pliocene. Recent distribution: Holarctic. Remark: The observed fossil and recent individual displayed a more convex shell than the general form and can most likely be assigned to the var. mamillanum.

Materials and methods 4

4.1 The recent malacofauna

Site selection and sampling

Site selection was based on the “Biologische waterkwaliteit in Vlaanderen 2007” map, created by the Vlaamse Milieu Maatschappij (VMM). The colour code attributed to the sites represents the biological water quality calculated as the Belgian Biotic Index (BBI). Within the catchment of the Moervaart and Zuidlede, the two main rivers flowing through the fossil Moervaartdepression, a total of 11 sites was selected (Fig. 6, Table 1): seven sites of good water quality (BBI = 7‐8, green triangles), two of moderate (BBI = 5‐6, orange triangles) and two of bad water quality (BBI = 3‐4, red triangles). The imbalance in the amount of sites per water quality category is due to the lack of moderate and bad quality sites in the studied area. Environmental parameters and GPS data were collected from the VMM website (www.vmm.be). Not all available environmental parameters were used, but only those of which data were available for most of the sites.

Site WQ Watercourse X Y 38020 good Moervaart 111221 203989 39000 good Moervaart 114477 206275 39800 good Moervaart 120625 207000 44000 good Langlede 113605 206766 52200 good Zuidlede 118929 204683 52700 good Zijarm Moervaart 111100 204312 52800 good Zijarm Moervaart 111057 204297 40800 moderate Oude Vaartbeek 122999 203748 53300 moderate Kapittelvaardeken 117382 204197 53500 bad Kapittelvaardeken 114732 200757 53550 bad Kleine Watergang 117288 201458

Table 1: Site number, biological water quality (WQ) and location of the recent sampling sites.

Fig. 6: Location of the recent sampling sites, A: within the Moervaartdepression, B: within the Flemish Valley (source: Ann Zwertvaeger, unpublished).

Sampling took place at the end of June and the beginning of September 2009. Vegetation and mud was collecting from the river bank with a dipping net and transferred onto a rectangular sieve. Stones, vegetation and branches were picked up by hand. All this material was carefully checked for molluscs. Attached molluscs were removed and put in jars. Vegetation, branches and stones were placed back in the water. The sampling strategy was different for site 53550 since the brook had dried out. Several kilograms of sediment were dug out around the remaining water plants and were sieved, but this yielded no snails. Collecting time was approximately two hours per site. According to Økland (1990) this should be sufficient to collect the full range of species present at a site. The best harvesting was reached among vegetation in shallow waters (down to ca. 1m). For more information on general sampling rules and sampling equipment we refer the reader to Økland (1990) and Boesveld et al. (2009a).

Data collecting

Taxonomical identification to species level was done by comparison with standard works. For freshwater molluscs Adam (1960), Devriese et al. (1997) and Glöer & Meier‐Brook (1994) were used. Molluscs collected from good quality locations were not identified at the site. During the time period necessary for data collecting, these molluscs were kept alive in separate aquaria per sampling site. The aquaria were placed in a well lighted spot but out of reach of direct sun light to prevent excessive heating and oxygen depletion of the water. Every week, rocks overgrown with algae were

36 placed in the aquaria as a food source. Keeping the molluscs alive helps identification, because it prevents contracting and hiding in the shells as often happens during fixation of recent specimens (Økland, 1990). After identification and counting, the molluscs were released in the Zwalm and Scheldt River at Nederzwalm. For the moderate and poor quality locations identification and counting was done at the site. The greatest attention was paid to living snails, but dead snails or shells were also inspected.

Statistical analysis

The dataset was evaluated by multivariate statistical analysis in R version 2.10.0 (© 2009 The R Foundation for Statistical Computing) using the package vegan (Oksanen et al., 2008). The variation in the dataset was first examined by means of a Detrended Correspondence Analysis (DCA; Hill, 1979). DCA reduces the “arch‐effect” (Hammer & Harper, 2006) and helps to decide whether a unimodal or linear response model is most appropriate for further application. Since the DCA1 axis length was larger than 2, Correspondence Analysis (CA, Jongman et al., 1995) and Canonical Correspondence Analysis (CCA, Legendre & Legendre, 1998) can be applied to the dataset. Canonical Correspondence Analysis is a good choice if the user has clear and strong a priori hypothesis. Since this was not the case in our study and we were interested only in the major structure of the dataset, we decided to perform CA and integrate the environmental parameters by means of environmental after analysis. Relationships between assemblage composition and environmental parameters were assessed by fitting vectors to the ordination using the function envfit or environmental fit. This function finds the direction in the ordination space towards which each environmental variable (or the abundance of each taxon) changes most rapidly and to which it is maximally correlated with the ordination configuration (Oksanen et al., 2008; Oksanen, 2010). The significance of the environmental vectors was determined by a permutation test (n=10000) of the environmental variables. Next to chemical (chloride, conductivity, oxygen content, etc.) and nutrient (nitrates, phosphates, sulphates, etc.) parameters, biological water quality (BBI) was added as a parameter to check whether water quality could be assessed based on molluscs alone. Some sampling sites had missing data for some of the selected environmental parameters. These missing data, indicated as “not available” (NA), were removed. Site 53550 could also not taken into account for statistical analysis since it yielded no snails at all.

4.2 The fossil malacofauna

For the purpose of the GOA project, a large trench was dug through the deepest part of the former lake at the end of March 2009. All fossil material was collected from this trench.

Sampling

Sampling took place at the end of March and the beginning of April 2009. Four samples in total were taken for molluscan investigation: three complete sequences (DVD S1, S2 and S4) and one sample of a local bivalve accumulation (DVD S3). Two sequences were selected for malacological investigation. One on the east side of the trench named DVD S2 (X = 119215, Y = 206684) and one on the west side, DVD S4 (X = 119222, Y = 206634). Both locations are situated in close proximity of each other, but even so, differences in thickness of the subsequent layers were observed.

The DVD S2 sequence, S2 for short, consists of 18 samples taken at a more or less regular interval of 10cm over a depth of 136 cm. In some cases sampling deviated from the chosen interval in order not to mix stratigraphic units. Every sample consisted of several kilograms of sediment and was kept in closed plastic bags to keep the sediment moist.

A similar strategy was followed for the DVD S4 sequence, S4 for short. Here, sampling was done according to the stratigraphic units as defined by the archaeologists and each sample matches one stratigraphic unit. Some units are only a couple of centimetres thick whereas others have a depth of 30 cm. In total the S4 sequence consists of 14 samples over a depth of 164 cm. For both sequences the entire depth of the deposits was sampled. The exact location of every site and sample was measured using GPS (standard deviation = 1.3 cm).

The digging of the trench and bad weather has washed out large amounts of shells. A fraction of these shells was collected with plain kitchen sieves. After washing and drying, large and unbroken specimens were picked out and kept in small cardboard trays labelled with the correct scientific name as reference material.

Laboratory procedures

In the laboratory, subsamples of 500ml were taken from the original bags and air‐dried. Once dried, each sample was divided into two parts: one quarter for palaeoecological analysis and three quarters for stable isotope analysis (4.3). Each palaeontological sample was put into a plastic cup, covered with water and gently stirred. Disintegration of the sediment took place and shells started to float to the surface. The floating shells were poured off into a coffee filter and left to dry. The residue was poured through a nest of sieves (mesh 1 mm, 0.5 mm and 150 micron) and wet sieved to remove the majority of sediment. Sediments such as organic clays are particularly resistant to disintegration.

Samples of this type were first dissolved in Na4P2O7 (Bates et al., 1978) before sieving the residue. Once both fractions had dried, identification and counting could begin.

Identification and counting

Most shells were medium to small sized and were examined with the aid of a Zeiss Stereo Discovery.V8 (PlanS 1.0x FWWD 81mm) binocular microscope. Identifications were based on the same standard works as mentioned in 4.1. Adam (1960) was especially helpful for the identification of the Pisidium species, because it is one of the few books which includes drawings of the hinge and teeth of these small bivalves. Identification of terrestrial snails was done with Kerney & Cameron (1980). Non‐apical fragments were excluded from counting and analysis, since several such fragments can derive from one single broken shell. For each sample, all identifiable shells and opercula were counted even if the sample contained more than a thousand individuals. This type of counting gives an indication of both the fossil mollusc community as well as the total molluscan biomass present in the samples. The shells and opercula of Bithynia tentaculata were grouped together, because some samples contained only shells or only opercula. For the Pisidium species, each valve was seen as an individual since grouping of left and right valves is difficult for such small bivalves.

Presentation of results and photography

The photographs used to make the Plate 1 & 2 were taken with optical magnification. For shells smaller than 1cm, a Canon EOS 350D camera was linked to the Zeiss Stereo Discovery.V8 binocular. Larger shells were photographed with a Canon EOS 350D camera with a EFS 60mm f/2.8 Macor USM macro‐lens. All photos were adapted in Adobe Photoshop CS2 9.0 (© 1990‐2005 Adobe Systems Incorporated).

Once all the samples had been processed, a species by sample table was constructed. From this table a percentage diagram, that is, a diagram displaying each species as a percentage of the total recovered shells in each sample was constructed using the C2 Version 1.5 (© 2003‐2007 University of Newcastle). The C2 diagram was edited in Adobe Illustrator CS 11.0.0 (© 1987‐2003 Adobe Systems Incorporated) and divided into molluscan zones according to the major faunal changes throughout the sequence.

Numerical analysis

The molluscan biozonation was further evaluated by multivariate statistical analysis in R version 2.10.0 (© 2009 The R Foundation for Statistical Computing) using the package vegan (Oksanen et al., 2008). The variation in the dataset was first examined by means of a Detrended Correspondence Analysis (DCA; Hill, 1979). As mentioned in 4.1, DCA reduces the “arch‐effect” (Hammer & Harper, 2006) and helps to decide whether a unimodal or linear response model is most appropriate for further application. The DCA1 axis length (> 2) for sequence S4 confirmed a unimodal response, resulting in the use of a Correspondence Analysis (CA). The CA plot can be interpreted as followed: (1) sites that lie close to each other have a similar species composition, (2) species that lie close to each other have a tendency to live together and (3) species in the proximity of a certain site are often present in that site. For the S2 sequence, the DCA1 axis length (<2) pointed towards a Principal Component Analysis (PCA). The output of the PCA‐analysis showed a very unusual plot: all samples and species, except one, were plotted in a straight line parallel to Axis 2 (not shown). Therefore, the S2 dataset was slightly modified and sample one which contained only one Sphaerium corneum was removed. DCA was rerun for the modified dataset and resulted in a DCA1 axis length > 2 and thus a CA (Jongman et al., 1995).

Taxonomic diversity in the dataset was examined by means of biodiversity indices. According to Hammer & Harper (2006) these indices give a quantitative estimate of the biodiversity of a community based on a sample from the once‐living community. Species richness (S) is the simplest possible index and represents the number of species present in a sample. The second calculated index is the Shannon‐Wiener index, which is a popular but more complex diversity index. Calculation of the index is done by means of the following equation.

H’ = ‐ ∑ pi ln pi

with pi = ni/n with ni = number of individuals of a particular species i and n= total number of individuals of all species

The Shannon‐Wiener index varies from zero for communities with only one single species to high values for communities with many taxa (Hammer et al., 2001). Evenness quantifies how equal the community is and can be derived from the Shannon‐Wiener index when H’ is divided by log(S). If all species in a sample are represented by more or less the same number of individuals, evenness will be great. If however one species dominates the sample, evenness will be small. Evenness is thus the opposite of dominance. The last index is the Simpson’s index of diversity 1‐D, which was calculated as follows.

2 1‐D = 1 ‐ ∑ pi

The Simpson index of diversity indicates the probability that two randomly picked individuals are of the same species (Hammer & Harper, 2006). The value of the index ranges between zero and one: the greater the value, the greater the sample diversity. The indices were calculated with the help of the Chang Bioscience Shannon‐Wiener Diversity Index/Shannon Entropy Calculator (www.changbioscience.com).

Furthermore, environmental indicator indices were calculated based on species with specific habitat preferences. The communities were screened for four factors: water temperature, the size of the water body, the occurrence of dry phases and the amount of water movement. The ratios were calculated using the following equations.

ௐ ௡ௐ ൌ with W = warm water indicator species ஼ ሺ௡ௐା௡஼ሻ C = cold water indicator species n = relative abundance ௅ ௡௅ ൌ with L = large water body indicator species ௌ ሺ௡௅ା௡ௌሻ S = small water body indicator species n = relative abundance

஽ ௡஽ ൌ with D = drought sensitive indicator species ோ ሺ௡஽ା௡ோሻ R = drought resistant indicator species n = relative abundance

ௌ௧ ௡ௌ௧ ൌ with St = stagnant water body indicator species ி ሺ௡ௌ௧ା௡ிሻ F = flowing water body indicator species n = relative abundance

The appointment of species to a certain group was based on the ecological requirements stated in Gittenberger & Janssen (2004). Warm water indicators are Acroloxus lacustris, Bithynia tentaculata and Lymnaea stagnalis; cold water indicators are Pisidium obtusale and Valvata piscinalis. Species with a preference for large surface waters are Acroloxus lacustris, Bithynia tentaculata, Lymnaea stagnalis, Myxas glutinosa, Physa fontinalis, Pisidium amnicum, Pisidium nitidum, Radix auricularia and Valvata piscinalis, whereas Pisidium milium, Pisidium pulchellum and Pisidium obtusale prefer

40 small surface waters. Drought resistant species are Galba truncatula, Pisidium casertanum, Planorbis planorbis, Radix balthica and Valvata cristata; drought sensitive species are Bathyomphalus contortus, Gyraulus crista, Pisidium milium, Pisidium nitidum and Sphaerium corneum. Pisidium amnicum, Pisidium milium, Pisidium pulchellum and Pisidium tenuilineatum preferentially inhabit flowing waters, while other species like Acroloxus lacustris, Anisus vortex, Gyraulus crista, Lymnaea stagnalis, Myxas glutinosa, Pisidium obtusale, Radix auricularia, Radix balthica and Valvata cristata prefer stagnant conditions.

4.3 Stable carbon and oxygen isotopes

Sample preparation

The general method for sample preparation was based on Apolinarska (2009) but slightly adapted.

Shells were not pre‐treated with H2O2 to eliminate the periostracum, because any type of pre‐ treatment with C or O containing reagents could possibly alter the isotopic composition of the shells.

Consequently, Na4P2O7 was also not applied.

After sieving and drying, shells were sorted per species and per sample. The largest individuals were picked out and checked for indication of corrosion or re‐crystallization with the help of a Zeiss Stereo Discovery.V8 (PlanS 1.0x FWWD 81mm) binocular microscope. Shells with impurities were discarded. Each specimen was cleaned thoroughly by repetitive washing and manual cleaning with a brush. Repeatedly dropping the shells on a desk or any other type of hard substrate from a height of 20 to 30cm helped loosen the sediments inside, without fractionating the shell. Cleaning in an ultrasonic bath was considered, but appeared not advisable for specimens smaller than 1 cm. The intensive cleaning procedure is necessary to eliminate any organic carbonates from sources other than molluscs, since even minute amounts may influence the isotopic signal (Prof. E. Keppens, pers. comm. 2010).

Afterwards cleaning, all samples were weighed with a high precision balance. Samples that did not reach the necessary minimum weight of 60 µg were discarded. In order to obtain the most detailed sequence of stable isotope measurements, sequence S2 and S4 were combined. Interpretation of the profiles of S2 and S4 allowed to correlate the samples. Samples 1 to 15 and sample 18 were taken from sequence S2 and were supplemented with samples 8 to 12 from sequence S4. From the total available samples, only those of species which were present throughout large parts of the sequence were kept for further analysis. The selected species were Bithynia tentaculata (shells and opercula), Gyraulus albus, Radix balthica and Valvata piscinalis. The last preparation step consisted of grinding the shells into a fine powder and dividing samples of more than 120 µg for multiple measurements.

Stable isotope measurement

All the steps in this procedure happen in vacuum, because air contains C and O containing gasses.

Within the vacuum, the calcareous powder reacts with phosphoric acid (H3PO4) at 70°C and form

CO2, which is then measured for its isotopic composition (Fig. 7). Afterwards, the sample isotopic composition is compared to a standard isotopic composition necessary for calculation of the δ18O and δ13C values. Results are reported relative to PDB (= Peedee Belemnite).

Fig. 7: Scheme of a simple mass spectrometer for measurement of the isotope ratios (IRMS) (source: http://pubs.usgs.gov/of/2001/ofr01‐257/index.html).

Stable isotope measurement was supposed to be performed with a ThermoFinnigan delta +XL mass spectrometer connected to a Kiel III device (Thermo Electron Corporation) at the Isotope Ratio Mass Spectrometry (IRMS) laboratory of the Vrije Universiteit Brussel (VUB). Unfortunately this was not possible due to technical problems. Running of standards at the end of March gave standard deviations of 0.4‰, which is ten times larger than what they are supposed to be. The problem was not resolved before the deadline of this thesis.

Results 5

Part I: The recent malacofauna

The dataset was analysed statistically to check which environmental parameters determine the presence or absence of molluscs and to verify what effect water quality has on the malacofauna. As for the fossil malacofauna, we focus on the communities and the individual species.

5.1 The recent species

In total, twenty species of freshwater molluscs were recovered. Sixteen species of gastropods and four species of bivalves. The recent community contains five species not present in the fossil material: three gastropod and two bivalve species.

Freshwater gastropods:

‐ Acroloxus lacustris (Linnaeus, 1758) ‐ Anisus vortex (Linnaeus, 1758) ‐ Bathyomphalus contortus (Linnaeus, 1758) ‐ Bithynia leachii (Sheppard, 1823) ‐ Bithynia tentaculata (Linnaeus, 1758) ‐ Gyraulus albus (Müller, 1774) ‐ Gyraulus crista forma cristata (Linnaeus, 1758) ‐ Hippeutis complanatus (Linnaeus, 1758) ‐ Lymnaea stagnalis (Linnaeus, 1758) ‐ Physa acuta (Draparnaud, 1805) ‐ Physa fontinalis (Linnaeus, 1758) ‐ Planorbarius corneus (Linnaeus, 1758) ‐ Planorbis planorbis (Linnaeus, 1758) ‐ Radix balthica (Linnaeus, 1758) ‐ Valvata cristata (Müller, 1774) ‐ Valvata piscinalis (Müller,1774)

Freshwater bivalves

‐ Dreissena polymorpha (Pallas, 1771) ‐ Musculium lacustre (Müller, 1774)

‐ Pisidium subtruncatum (Malm, 1855) ‐ Sphaerium corneum (Linnaeus, 1758)

5.2 Statistical analysis

Detrended Correspondence Analysis (DCA) produced a DCA1 axis length of 3.0521. As we already mentioned, an axis length larger than two standard deviations indicates that unimodal statistical techniques are most appropriate for further analysis and thus Correspondence Analysis (CA) was performed. CA gives an insight into the ecological similarities and differences between sites and species and yielded a good spreading of the data. The first and second CA axes explain respectively 32.1% and 19.6% of the total variance; together this amounts to 51.7% of the total variance explained by the first two axes. In contrast to the majority of species, Physella acuta is plotted on the positive site of CA1 (Fig. 8). The variation explained by CA2 plots Gyraulus albus on the positive side and Physa fontinalis and Pisidium subtruncatum on the negative side. Four good quality sites and one moderate quality site are located close to each other within the left upper quadrant of the CA plot. The other sampling points have a more remote position.

CA1 CA2 r2 Pr(>r) WQ ‐0.999726 0.023399 0.998 1.00E‐04 *** Cl ‐0.845368 0.534185 0.9184 0.06759 * N3 0.992229 0.124422 0.9729 0.0337 ** N2 0.980051 0.198744 0.8065 0.21908 PO 0.947661 ‐0.319279 0.675 0.39996 SO 0.863366 ‐0.504577 0.7083 0.37696 Pt 0.9962 ‐0.087099 0.3063 0.71823 Ov ‐0.621364 0.783522 0.4949 0.49895 EC ‐0.13473 0.990882 0.3175 0.59344 O2 ‐0.655322 0.75535 0.4869 0.49895 pH ‐0.263959 0.964534 0.8248 0.24068 ZS ‐0.631649 0.775255 0.4934 0.51395 NH 0.998796 0.049057 0.5868 0.36766 KN 0.949494 0.313785 0.5998 0.40186 BZ 0.028962 0.999581 0.7198 0.26147 CZ ‐0.81864 0.574306 0.8927 0.09639 *

Table 2: Correlation between environmental vectors and the ordination plot from correspondence analysis. Significance codes are: *** <0.001, ** <0.05, * <0.1. The environmental parameters are: WQ: biological water quality, Cl: chloride, N3: nitrates, N2: nitrites, PO: orthophosphates, SO: sulphates, Pt: total phosphates, Ov: oxygen saturation, EC: conductivity, O2: dissolved oxygen, pH: hydrogen‐ion concentration, ZS: suspended solids, NH: ammonia, KN: Kjeldahl nitrogen, BZ: biological oxygen demand after 5 days, CZ: chemical oxygen demand.

The environmental parameters were included in the CA plot by means of environmental fit (envfit) and are indicated by arrows. The statistical significances (Pr(>r)) or p‐values indicate that four environmental parameters correlate significantly with the first two axes of the CA ordination

44 diagram. These parameters are biological water quality (WQ), chloride (Cl), nitrates (N3) and chemical oxygen demand (CZ). Biological water quality is strongly linked to axis 1 and displays a gradient from good quality sites to poor quality sites from left to right. The amount of nitrates is also strongly linked to axis 1 and increase as the water quality decreases. The squared correlation coefficients (r2) evaluate the strength of the relationship: the higher the value, the stronger the relationship. For the significant environmental parameters the r2 values indicate a strong relationship.

Fig. 8: Ordination diagram of the first two axes of correspondence analysis for molluscan communities of 10 recent sampling sites. The arrows represent environmental gradients fitted to the ordination space: WQ: biological water quality, Cl: chloride, CZ: chemical oxygen demand, N3: nitrates. Sites are represented by numbers colour coded according to their water quality: good (green), moderate (orange) and bad (red). Species names were abbreviated to a three letter code consisting of the first letter of the genus name and the first two letters of the species name. The species present are: Ala: Acroloxus lacustris, Avo: Anisus vortex, Bco: Bathyomphalus contortus, Ble: Bithynia leachii, Bte: Bithynia tentaculata, Dpo: Dreissena polymorpha, Gal: Gyraulus albus, Gcr: Gyraulus crista, Hco: Hippeutis complanatus, Lpa: Lymnaea palustris, Lst: Lymnaea stagnalis, Mla: Musculium lacustre, Pac: Physa acuta, Pfo: Physa fontinalis, Psu: Pisidium subtruncatum, Pco: Planorbarius corneus, Ppl: Planorbis planorbis, PteRba: Radix balthica, Sco: Sphaerium corneum, Vcr: Valvata cristata, Vpi: Valvata piscinalis.

Part II: The fossil malacofauna

The fossil dataset will be analysed in various ways in order to obtain as much information as possible. The focus is put on the communities and the individual species. We start by giving a list of the recovered species, followed by a separate description of both sequences.

5.3 The fossil species

In total, thirty two species of molluscs were retrieved from the sequences S2 and S4: twenty three species of gastropods and nine species of bivalves. Among these species twenty five belong to freshwater molluscs and seven to terrestrial ones. Bithynia tentaculata was the only species of which both shells and opercula were found. The Valvata species also have an operculum but we found none of these, perhaps because of their small size. The Vertigo sp. could not be identified to species level, because the last whorl of the shell had broken off. Within the genus Vertigo, identification is based on the apertural teeth. Fragmentary Pisidium shells and juveniles with not fully developed hinges were not taken into account. Freshwater gastropods found are:

‐ Acroloxus lacustris (Linnaeus, 1758) ‐ Anisus vortex (Linnaeus, 1758) ‐ Bathyomphalus contortus (Linnaeus, 1758) ‐ Bithynia tentaculata (Linnaeus, 1758) ‐ Galba truncatula (Müller, 1774) ‐ Gyraulus albus (Müller, 1774) ‐ Gyraulus crista forma cristata (Linnaeus, 1758) ‐ Hippeutis complanatus (Linnaeus, 1758) ‐ Lymnaea stagnalis (Linnaeus, 1758) ‐ Myxas glutinosa (Müller, 1774) ‐ Physa fontinalis (Linnaeus, 1758) ‐ Planorbis planorbis (Linnaeus, 1758) ‐ Radix auricularia (Linnaeus, 1758) ‐ Radix balthica (Linnaeus, 1758) ‐ Valvata cristata (Müller, 1774) ‐ Valvata piscinalis (Müller,1774)

Freshwater bivalves are:

‐ Pisidium amnicum (Müller, 1774) ‐ Pisidium casertanum (Poli, 1791) ‐ Pisidium milium (Held, 1836) ‐ Pisidium nitidum (Jenyns, 1832) ‐ Pisidium obtusale (Lamarck, 1818) ‐ Pisidium pulchellum (Jenyns, 1832) ‐ Pisidium subtruncatum (Malm, 1855) ‐ Pisidium tenuilineatum (Stelfox, 1918)

‐ Sphaerium corneum (Linnaeus, 1758)

Terrestrial species are:

‐ Carychium minimum (Müller, 1774) ‐ Punctum pygmaeum (Draparnaud, 1801) ‐ Pupilla muscorum (Linnaeus, 1758) ‐ Succinella oblonga (Draparnaud, 1801) ‐ Succinea putris (Linnaeus, 1758) ‐ Vallonia pulchella (Müller, 1774) ‐ Vertigo sp.

5.4 The S2 sequence

5.4.1 Relative abundance and molluscan zonation

5.4.1.1 Correspondence Analysis

As we already mentioned in 4.1, sample 1 was omitted from statistical analysis. The removal of this sample led to a DCA1 axis length of 2.1556. An axis length larger than two indicates that unimodal statistical techniques are most appropriate for further analysis and thus Correspondence Analysis (CA) was performed. CA gives an insight into the ecological similarities and differences between samples and species. The first and second CA axes explained respectively 32.2% and 27.2% of the total variance; together this adds up to 59.4% of the total variance explained by the first two axes. The definition of the molluscan zones was determined based on statistical analysis and visual interpretation. Distribution of the samples on the CA plot allowed to identity three clusters of samples (Fig. 9): cluster 1 consists of samples 18 to 14, cluster 2 is composed of samples 13 to 11 and cluster 3 contains samples 5 and 4. All other samples could not be grouped and remained separate. Sample one was also regarded as a separate zone. In total 10 zones can be distinguished, they are labelled ZA to ZI from the bottom to the top of the profile. The position of sample seven at the edge of the CA plot needs to be interpreted with caution. The eccentric position of this sample can be due to the fact that it contains two species, Acroloxus lacustris and Radix auricularia, that are exclusively found in this sample. Statistical uncertainty may also have caused the eccentric position. The close positioning of samples two and ten to respectively cluster one and two indicate that the community of these samples largely resemble the communities the respective clusters. Both samples could not be included in the respective clusters because they are not adjacent to any of the samples in these clusters. The same goes for sample 8 and 6. The communities of cluster two and three are dominated by Bithynia tentaculata, whereas freshwater bivalves are particularly present in samples 9 to 6 and sample 3.

Fig. 9: Ordination diagram of the first two axes of correspondence analysis for the molluscan communities of sequence S2. Samples are indicated by black diamonds and species by red dots. Every species point is fitted with a three‐letter code composed of the first letter of the genus name and the first two letters of the species name. Sample numbers increase from top to bottom and sample one was omitted from analysis. Grey zones group samples that are part of the same molluscan zone and are labelled with the letter of this zone. The species present are: Ala: Acroloxus lacustris, Avo: Anisus vortex, Bco: Bathyomphalus contortus, Bte: Bithynia tentaculata, Gal: Gyraulus albus, Gcr: Gyraulus crista, Hco: Hippeutis complanatus, Lst: Lymnaea stagnalis, Mgl: Myxas glutinosa, Pfo: Physa fontinalis, Pam: Pisidium amnicum, Pca: Pisidium casertanum, Pmi: Pisidium milium, Pni: Pisidium nitidum, Pob: Pisidium obtusale, Ppu: Pisidium pulchellum, Psu: Pisidium subtruncatum, Pte: Pisidium tenuilineatum, Ppl: Planorbis planorbis, Rau: Radix auricularia, Rba: Radix balthica, Sco: Sphaerium corneum, Spu: Succinea putris, Vcr: Valvata cristata, Vpi: Valvata piscinalis.

5.4.1.2 Molluscan zonation (Fig. 10)

Molluscan zone A (137‐93 cm depth, samples 18 to 14): Gyraulus crista – Radix balthica zone The community of zone A is dominated by four species: Gyraulus albus, Gyraulus crista, Hippeutis complanatus and Radix balthica. These are the only species present at the base of the sequence. Higher up more gastropod species join the community as well as several freshwater bivalves (Pisidium casertanum, Pisidium milium, Pisidium pulchellum, Pisidium subtruncatum and Sphaerium corneum). There abundance gradually decreases towards the top of this zone. One terrestrial species Succinea putris was found but only in small percentage.

Molluscan zone B (93‐73 cm depth, samples 13 to 11): Bithynia tentaculata – Gyraulus crista zone Zone B has the highest molluscan biomass of the whole sequence. Bithynia tentaculata and Valvata piscinalis appear for the first time in this zone. From the base of this zone Bithynia tentaculata is overwhelmingly dominant. Freshwater bivalves are virtually absent from these layers, a few individuals of Pisidium milium and Sphaerium corneum excepted. Planorbis planorbis appears in the upper most part of this zone.

Molluscan zone C (73‐70 cm depth, sample 10): Gyraulus crista – Gyraulus albus zone Zone C is characterised by a steep decrease in Bithynia tentaculata which is replaced by Gyraulus albus, Gyraulus crista and Myxas glutinosa. Planorbis planorbis disappears again in this zone. The major decline in the total molluscan biomass observed coincides with a change in sediment type. While zone B is characterised by marly deposits, sediments in zone C consist mainly of more or less peaty sediments alternating with thin aeolian sand layers. Despite the decrease in biomass, zone C is characterised by a high species diversity: a total of fifteen species was recovered. Pisdium species, like Pisidium casertanum, Pisidium milium, Pisidium nitidum, Pisidium pulchellum and Pisidium subtruncatum start to re‐enter the community.

Molluscan zone D (70‐63 cm depth, sample 9): Gyraulus crista – Valvata cristata zone Bithynia tentaculata almost completely disappears in zone D. The generalist Pisidium species, Pisidium casertanum, Pisdium milium and Pisidium subtruncatum, gain importance and take up one quarter of the community. Valvata cristata and Gyraulus crista are the most dominant species followed by the earlier mentioned Pisidium species. Hippeutis complanatus and Physa fontinalis disappear at the end of this zone. Both species will return in a later phase, but never as abundant as they did before.

Molluscan zone E (63‐59 cm depth, sample 8): Bithynia tentaculata – Valvata cristata zone After the steep decrease in zone E Bithynia tentaculata dominates again and is followed in abundance by Valvata cristata and Myxas glutinosa. The Pisidium species of zone D become less abundant or even absent. Zone E represents the last part of the stratigraphic column containing organic deposits and has the lowest species richness. Bathyomphalus contortus is confined to this zone.

Molluscan zone F (59‐52 cm depth, sample 7): Valvata piscinalis – Myxas glutinosa zone The change in substrate from organic sand to peat deposits in zone F coincides with a major change in molluscan community: the bivalves gain in importance whereas the gastropods decrease. Each group of species takes up half of the community. For the freshwater bivalves this is the highest relative abundance they reach in sequence S2. The gastropod community is composed of four frequent species (Valvata piscinalis, Myxas glutinosa, Pisidium milium, Pisidium subtruncatum) and several other less abundant species. Among these uncommon ones are Acroloxus lacustris and Radix auricularia, two species which are only present in zone F.

Molluscan zone G (52‐39 cm depth, sample 6): Bithynia tentaculata – Gyraulus albus zone The four main gastropods of zone F remain abundant in zone G. Not only Bithynia tentaculata increases in abundance but the re‐appearing Gyraulus albus also becomes an important part of the community. The Pisidium species which dominated zone F become less abundant. Two new species

50 enter the community in zone G, namely Anisus vortex and Pisidium tenuilineatum, to disappear again in zone H.

Molluscan zone H (39‐19 cm depth, sample 5 and 4): Bithynia tentaculata – Gyraulus crista zone Bithynia tentaculata is the most abundant species and reaches a maximum relative abundance of 80%, which is the highest value obtained from any species in sequence S2. Since Bithynia tentaculata takes up such a large part of the communities all other species are relatively less abundant. Several species of zone G disappear in zone H, e.g. the Pisidium species. Lymnaea stagnalis and Physa fontinalis disappear in the second part of this zone.

Molluscan zone I (19‐9 cm depth, sample 3 and 2): Bithynia tentaculata – Pisidium species zone The community of sample 3 largely resembles the community of zone G, except the Pisidium species take up only one third of the community. Sample 2 represents a thin peat layer at the top of the sequence and contains only two individuals: one of Bithynia tentaculata and one of Gyraulus crista. All the species of zone I disappear at the end of this zone.

Molluscan zone J (9‐0 cm depth, sample 1): Sphaerium corneum zone Only one individual of Sphaerium corneum was recovered from zone J, which represents an isolated layer of marl in between the peat of Zone I and the modern soil (not sampled here). This layer was not present throughout the whole of the trench. It is lacking in sequence S4 (see further).

5.4.2 Absolute abundance

The absolute abundances of the species found in the S2 sequence are shown in three different diagrams (Fig. 11 to 13). The species were divided into groups according to their appearance in the profile. Group A (Fig. 11) consists of species that are mainly present in the lower part of the sequence (136 – 63 cm depth, samples 18 to 9). The species of Group B (Fig. 12) are mostly present within the upper part of the sequence (93 – 0 cm depth, samples 13 to 1). Group C (Fig. 13) contains taxa that are present throughout the whole sequence. Species which were present only once or twice throughout the sequence were not taken into account. They represent only a small fraction of the total absolute abundance. These species are: Acroloxus lacustris, Anisus vortex, Bathyomphalus contortus, Pisidium amnicum, Pisidium nitidum, Pisidium tenuilineatum, Planorbis planorbis, Radix auricularia and Succinea putris.

Group A:

The lower part of sequence S2 is dominated by five species, namely Gyraulus albus, Gyraulus crista, Hippeutis complanatus, Physa fontinalis and Radix balthica. All species except Radix balthica display a similar trend in absolute abundance characterised by two peaks: the first between 122 cm and 93 cm and the second between 93 cm and 70 cm. Radix balthica displays a gradual increase in absolute abundance between 115 cm and 104 cm followed by a period of stasis from 104 cm to 93 cm. The number of Radix balthica shells increases again and reaches its maximum abundance between 93 cm and 70 cm. From approximately 70 cm upwards all species decline. They remain present but in small amount, that is, less than 20 individuals per sample.

Group B:

Group B is composed of only four species, namely Bithynia tentaculata, Myxas glutinosa, Pisidium obtusale and Valvata cristata. These species do not display a similar pattern in absolute abundance as those of Group A. Bithynia tentaculata reaches its peak abundance first, that is, between 93 cm and 73 cm. After a steep decline between 73 and 70 cm, Bithynia tentaculata remains present in the community but in much smaller amount. Myxas glutinosa, Valvata cristata and Pisidium obtusale display a pattern with multiple peaks. For Myxas glutinosa and Valvata cristata the first peak coincides between 82 cm and 70 cm, but afterwards these species complement each other, that is, if Myxas glutinosa increases Valvata piscinalis decreases and vice versa. Pisidium obtusale is the last species to enter the community in this group. It displays two peaks: a first large one between 59 cm and 39 cm and a second smaller one between 19 cm and 4 cm.

Group C:

Group C is characterised by species with multiple peaks throughout the sequence. For the Pisidium species a total of three major peaks can be defined. The first peak is located between 122 cm and 115 cm, the second and largest peak between 59 cm and 39 cm and the third peak between 19 cm and 4 cm. Valvata piscinalis displays a similar pattern, that is, the optima are located at the same depth. Sphaerium corneum is mostly present when other species are lacking, except for the peak between 59 cm and 39 cm. Lymnaea stagnalis is present throughout most of the sequence but always in small amount. The position of the major peaks of Group C coincide with the major peaks in Group A and B.

Fig. 11: Absolute abundance of species with a prevailing occurrence in the lower part of sequence S2 (136 – 63 cm depth, samples 18 to 9).

Fig. 12: Absolute abundance of species with a prevailing occurrence in the upper part of sequence S2 (93 – 0 cm depth, samples 13 to 1).

Fig. 13: Absolute abundance of species that occur throughout the whole sequence S2.

5.4.3 Species diversity

The Shannon‐Wiener diversity index and the Simpson’s diversity index display a similar pattern (Fig. 14). The indices show a small but gradual increase between 136 cm and 52 cm (ZA to ZF). The increase is not linear but shows some minor fluctuations. Within the same part of the sequence the number of species or species richness shows larger fluctuations. The largest species richness can be found in molluscan zone F. Between 136 cm and 39 cm (ZA to ZG) evenness stays more or less the same, around 0.75. All diversity indices display a large decline between 52 cm and 19 cm (ZG to ZH). This phase is followed by one last peak in diversity (ZI) after which the indices decreases again (ZJ), while evenness stays high.

Fig. 14: From left to right: Shannon‐Wiener diversity index, Simpson’s index of diversity, species richness and evenness throughout sequence S2.

5.4.4 Environmental indices

The size of the water body, the amount of water movement and temperature display a more or less simultaneous pattern (Fig. 15). The beginning of the sequence (136‐129 cm, sample 18, ZA) is characterised by low values of the temperature and size indices, and high values of the movement index. Between 129 cm and 90 cm (samples 17 to 13, ZA and ZB) the size and temperature indices increase while the movement index does not appear to change. This stage is followed by a stable period between 90 and 70 cm (samples 12 to 10, ZB and ZC) in which the environmental indices display high and constant values for temperature, size and movement. From 70 cm to 39 cm (samples 9 to 7, ZD to ZF) the sequence is characterised by strong fluctuations in the environmental indices. A drop in the temperature index is combined with a decrease in the size and movement indices and vice versa. Afterwards the environmental indices stabilise again between 39 cm and 19

54 cm (samples 6 to 4, ZG and ZH). The fossils recovered from these deposits display high values for size and temperature indices. The last major change takes place between 19 cm and 4 cm (samples 3 and 2, ZI): the temperature, size and movement indices exhibit one last negative fluctuation. The general decreasing trend between 4 cm and 0 cm (sample 1, ZJ) should be taken with caution as it might not reflect a true change in the environment, because only one individual of Sphaerium corneum was present in this sample.

The susceptibility towards drought displays a somewhat different pattern than the other indices. Between 136 cm and 70 cm (samples 18 to 10, ZA to ZC) the drought index has a high and more or less constant value, after which it decreases steeply between 70 cm and 59 cm (sample 9 and 8, ZD and ZE). Between 59 and 29 cm (samples 7 to 5, ZF to ZH) the index increases again. A last decline in the drought index is observed between 29 cm and 4 cm (samples 4 to 2, ZH and ZI). The observed value of the index between 4 cm and 0 cm (sample 1, ZJ) is doubtful considering that the only species found, Sphaerium corneum, is sensitive to drought.

Environmental indices 1.00 0.90 0.80 0.70 0.60 0.50 Drought 0.40 Size 0.30 Movement 0.20 0.10 Temperature 0.00 0 4 9 19 29 39 52 59 63 70 73 82 90 93 104 115 122 129 Depth (cm)

Fig. 15: Environmental indices of sequence S2: susceptibility to drought (red), size of the water body (green), amount of water movement (blue) and temperature (orange).

5.5 The S4 sequence

5.5.1 Relative abundance and molluscan zonation

5.5.1.1 Correspondence Analysis

Detrended Correspondence Analysis produced a DCA1 axis length of 3.9738. As we already mentioned, an axis length larger than two indicates that unimodal statistical techniques are most appropriate for further analysis and thus Correspondence Analysis (CA) was performed. The first and second CA axis explained respectively 36.5% and 24.7% of the total variance; together this amounts to 61.2% of the total variance explained by the first two axes. Two clusters of samples are visible (Fig.

16): cluster 1 consists of samples 13 to 8 and cluster 2 is composed of samples 7 and 6. The other samples are all regarded as individual zones. In total 7 zones have been identified namely za1 to zg from bottom to top. The remote position of sample 1 is probably due to the fact that it is the only zone which contains terrestrial species. The close positioning of sample 14 to cluster 1 indicates that the community of sample 14 largely resembles the communities in cluster 1. Sample 14 was not included in cluster 1 because it contained only three fractionated individuals of Gyraulus crista. The freshwater bivalves are mainly present in samples 5 and 3.

Fig. 16: Ordination diagram of the first two axes of correspondence analysis for the molluscan communities of sequence S4. Samples are indicated by black diamonds and species by red dots. Every species point is fitted with a three‐letter code composed of the first letter of the genus name and the first two letters of the species name. Sample numbers increase from top to bottom. Grey zones group samples that are part of the same molluscan zone and are labelled with the letter of this zone. The species present are: Ala: Acroloxus lacustris, Avo: Anisus vortex, Bco: Bathyomphalus contortus, Bte: Bithynia tentaculata, Cmi: Carychium minimum, Gtr: Galba truncatula, Gal: Gyraulus albus, Gcr: Gyraulus crista, Hco: Hippeutis complanatus, Lst: Lymnaea stagnalis, Mgl: Myxas glutinosa, Pfo: Physa fontinalis, Pam: Pisidium amnicum, Pca: Pisidium casertanum, Pmi: Pisidium milium, Pni: Pisidium nitidum, Pob: Pisidium obtusale, Ppu: Pisidium pulchellum, Psu: Pisidium subtruncatum, Ppy: Punctum pygmaeum, Pupilla muscorum, Sob: Succinella oblonga, Rba: Radix balthica, Sco: Sphaerium corneum, Vpu: Vallonia pulchella, Vcr: Valvata cristata, Vpi: Valvata piscinalis, Vsp: Vertigo sp.

5.5.1.2 Molluscan zonation (Fig. 17)

Molluscan zone a1 (164‐157 cm depth, sample 14): Gyraulus crista zone The sands at the bottom of the sequence contained only three individuals of Gyraulus crista. The definition of this layer as a separate zone can be discussed, but we kept is as a distinctive zone because we are not sure whether the material was original or reworked.

Molluscan zone a2 (157‐110 cm depth, sample 13 to 8): Gyraulus crista – Radix balthica zone Zone a2 is characterised by the presence of a diversified community. In total 13 species appear for the first time. Gyraulus albus, Gyraulus crista and Radix balthica dominate in this zone. The other freshwater gastropods are Hippeutis complanatus, Lymnaea stagnalis, Myxas glutinosa, Physa fontinalis, Valvata cristata and Valvata piscinalis. The amphibious gastropod Galba truncatula is only present in the oldest sandy part of zone a2.

Molluscan zone b (110‐80 cm depth, sample 7 and 6): Bithynia tentaculata – Gyraulus crista zone The three major characteristics of zone b are a spectacular increase in the number of shells, an almost complete absence of freshwater bivalves, and the first appearance and immediate dominance of Bithynia tentaculata. All species reach higher numbers than they did before, but their relative abundance decreases due to the invasion of Bithynia tentaculata. Bathyomphalus contortus is present in the lower half of the zone, but immediately disappears in the upper part.

Molluscan zone c (80‐74 cm depth, sample 5): Valvata piscinalis – Valvata cristata zone Zone c shows a steep decline in molluscan biomass. Several gastropod species disappear, namely Gyraulus albus, Hippeutis complanatus, Lymnaea stagnalis, Physa fontinalis and Radix balthica. Their place is taken by several Pisidium species and Valvata piscinalis. Bithynia tentaculata is still the most abundant species although its dominance is less pronounced as in the former zone.

Molluscan zone d (74‐67 cm depth, sample 4): Bithynia tentaculata – Valvata cristata zone Zone d represents the most species poor communities found in sequence S4 with the exception of zone a1. The community consists of only nine species of which Bithynia tentaculata is the most dominant with a relative abundance of 69%. The other major components of zone d are Valvata cristata and Myxas glutinosa. Acroloxus lacustris and Anisus vortex are only present in this zone. Molluscs recovered from this thin peat layer were often highly fragmented and unidentifiable, probably due to peat compression.

Molluscan zone e (67‐51 cm depth, sample 3): Valvata piscinalis – Pisidium species zone A large setback in Bithynia tentaculata characterises zone e. Gyraulus crista, Myxas glutinosa and Valvata piscinalis, along with several Pisidium species seem to profit from this situation. Numerous species which were absent during zones c and/or d reappear, namely Gyraulus albus, Hippeutis complanatus, Lymnaea stagnalis, Pisdium amnicum, Pisidium obtusale, Pisidium pulchellum, Radix balthica, Sphaerium corneum and Valvata piscinalis. Freshwater bivalves cover almost half of the community (48%), the largest relative abundance reached throughout the sequence.

Molluscan zone f (51‐21 cm depth, sample 2): Bithynia tentaculata – Valvataa piscinalis zone After a steep decline in zone e, Bithynia tentaculata dominates in zone f, and is followed by Valvata piscinalis and Gyraulus crista. All Pisidium species as well as Myxas glutinosa and Valvata piscinalis show a decline in relative abundance. The majority of species disappears after this zone.

Molluscan zone g (21‐0 cm depth, sample 1): Terrestrial snail zone The molluscs recovered from the modern soil point towards a shift from an aquatic to a terrestrial environment. The few freshwater species found are Bithynia tentaculata, Valvata cristata and Pisidium nitidum. The terrestrial community is built up of six species listed in order of dominance: Succinella oblonga, Punctum pygmaeum, Vallonia pulchella, Carychium minimum, Pupilla muscorum and Vertigo sp. The total number of shells is one of the lowest in the sequence.

5.5.2 Absolute abundance

As for sequence S2, the results of the S4 sequence expressed in absolute abundance are presented in three different diagrams (Fig. 18 to 20) corresponding to three groups of species (D to F) defined according to their appearance in the profile.

Group D (Fig. 18):

Group D represents species which dominate the lower part of the diagram (164‐80 cm, samples 14 to 6, za1 to zb). All the species of this group display one or more peaks in abundance between 164 cm and 80 cm depth (samples 14 to 6, za1 to zb). Gyraulus albus, Gyraulus crista, Hippeutis complanatus, Physa fontinalis and Radix balthica reach their highest absolute abundance simultaneously between 110 cm and 80 cm (samples 7 and 6, zb). Only Sphaerium corneum deviates from this pattern. It attains its peak abundance earlier, that is, between 130 cm and 96 cm (samples 8 and 7, za2 and zb). From 80 cm upwards, most taxa remain present in the community but in much smaller numbers. P. fontinalis even disappears completely above 80 cm depth. Not all taxa are present from the beginning of the sequence. G. crista is the first species to appear, followed by G. albus and R. balthica/peregra. G. crista is the most abundant species in this first phase (164‐110 cm, samples 14 to 8, za1 and za2) and displays a first peak between 147 cm and 133 cm (samples 12 to 10, za2). From 147 cm onwards H. complanatus, P. fontinalis and S. corneus gradually enter the community.

Group E (Fig. 19):

Group E is composed of species that are mainly present in the upper part of sequence S4 (110‐0 cm. All Pisidium species display a simultaneous peak between 67 cm and 21 cm depth (samples 3 and 2, ze and zf). This optimum corresponds to a peak in relative abundance at the same depth. Some of these species appear before 67 cm, but never in large numbers. The trend in the absolute abundance of Bithynia tentaculata is different from the Pisidium species. The maximum abundance of B. tentaculata is reached between 110 cm and 80 cm (samples 7 and 6, zb). After reaching its maximum abundance, B. tentaculata remains present in the community but in much smaller amount. Valvata cristata displays a completely different pattern than all the other species in this group. The maximum abundance of Valvata cristata is located at 96 cm to 67 cm (samples 6 to 4, zb to zd) in between the optima of B. tentaculata and the Pisidium taxa. From 67 cm onwards V. cristata gradually declines.

Fig. 18: Absolute abundance of species with a prevailing occurrence in the part half of the sequence S4 (164‐80 cm, samples 14 to 6, za1 to zb).

Fig. 19: Absolute abundance of species with a prevailing occurrence in the upper part of the sequence S4.

Group F (Fig. 20):

The three species in Group F are present throughout the sequence. All the species display two major peaks throughout the sequence. The first optimum is located between approximately 110 cm and 80 cm (samples 7 and 6, zb), the second optimum between 67 cm and 21 cm (samples 3 and 2, ze and zf). Lymnaea stagnalis reaches its maximum abundance within the first peak, whereas Myxas glutinosa and Valvata piscinalis are abundant within the second peak. Before, in between and after these two peaks the snails are much less abundant or sometimes even absent.

Fig. 20: Absolute abundance of species that occur throughout the sequence S4.

5.5.3 Species diversity

The Shannon‐Wiener diversity index, the Simpson’s diversity index and the evenness display a similar pattern (Fig. 21) between 164 cm and 51 cm (samples 14 to 3, za1 to ze). All indices show a steady increase between 164 cm and 130 cm (samples 14 to 9, za1 and za2) followed by a plateau between 130 cm and 51 cm (samples 8 to 3, za2 to ze). Species richness shows an augmentation from the bottom of the profile to 130 cm depth (samples 14 to 9, za1 and za2), followed by a plateau between 130 cm and 96 cm (samples 8 and 7, za2 and zb) and a decline from 96 cm to 67 cm (samples 6 and 4, zb to zd). The species richness increases for the last time between 67 cm and 51 cm (sample 3, ze) and reaches a maximum at the end of zone e. According to the Shannon‐Wiener diversity, the Simpson’s diversity and the evenness the maximum diversity can be found at the end of zone c. From 51 cm onwards (samples 2 and 1, zf and zg), the Shannon‐Wiener diversity, Simpson’s diversity and species richness gradually decrease whereas the evenness remains more or less unchanged, after a short decrease in zone e.

Fig. 21: From left to right: Shannon‐Wiener diversity index, Simpson’s index of diversity, species richness and evenness throughout sequence S4.

5.5.4 Environmental indices

The temperature, size and movement indices display a similar pattern throughout the profile (Fig. 22). The beginning of the sequence at 164 cm depth is characterised by very low temperature and size indices. Between 147 cm and 133 cm (samples 12 to 10, za2) the size index steeply increases. At the same time the values of the movement index remain more or less the same. The temperature index show a different pattern: after a first increase between 147 and 142 cm (sample 13, za2) the index decreases from 142 cm to 133 cm (samples 12 and 11, za2). This first phase (164‐133 cm, samples 14 to 11, za1 and za2) is followed by a period of stasis between 133 and 110 cm (samples 9 and 8, za2) in which the environmental indices remain more or less the same. The temperature index increases steeply between 110 cm and 80 cm (samples 7 and 6, zb) while the other indices remain constant. From 80 cm upwards the sequence is characterised by strong fluctuations in the environmental indices: a drop in temperature index is combined with a decrease in size and movement indices, and vice versa. The biggest decrease in temperature index is observed at 51 cm depth. The observed value of the temperature, size and movement indices between 21 cm and 0 cm (sample 1, zg) are doubtful considering that the community contained only three freshwater species, namely Bithynia tentaculata, Pisidium nitidum and Valvata cristata.

The susceptibility towards drought displays a somewhat different pattern than the other parameters. Between 164 cm and 133 cm (samples 14 to 10, za1 and za2) the community displays high index values. The observed value of the index at the bottom of the profile (164‐157 cm, sample 14, za1) is doubtful considering that the only species found, Gyraulus crista, is sensitive to drought. Between 133 cm and 67 cm (samples 9 to 5, za2 to zc) the drought index gradually decreases and reaches a

62 minimum value at the end of zone c. The drought index augments one last time between 67 cm and 21 cm (samples 4 to 2, zd to zf) and decreases in zone g (21‐0 cm, sample 1).

Environmental indices 1 0.9 0.8 0.7 0.6 Drought 0.5 0.4 Size 0.3 Movement 0.2 Temperature 0.1 0 0 21 51 67 74 80 96 110 130 133 137 142 147 157 Depth (cm)

Fig. 22: Environmental indices of sequence S4: susceptibility to drought (red), size of the water body (green), amount of water movement (blue) and temperature (orange).

Discussion 6

6.1 The recent malacofauna

The intention of the study of the recent malacofauna was to check whether the fossil and recent communities resembled each other. For several decades, freshwater ecosystems have been strongly affected by input of pollutants, shoreline alterations and water level reduction. Increasing anthropogenic impact is likely detrimental for the present‐day malacofauna and thus we expected to find a somewhat impoverished community. This was not entirely the case (see 6.4): most sites of good water quality displayed fairly rich communities. However, some displayed signs of habitat deterioration since 2007 and therefore we decided to extend the study to moderate and bad quality sites to check whether a link between biological water quality, environmental parameters and molluscan community exists.

Correspondence Analysis (CA) and environmental fit (envfit) show a significant relationship for three environmental parameters (nitrates, chloride, chemical oxygen demand) of which eutrophication by nitrates is the most important. Furthermore, the CA plot displays good spreading of good, moderate and bad quality sites and strong linkage with water quality, which indicates that molluscs may be useful indicators for water quality assessment. Four good quality sites (52200, 52800, 39000, 38020) and one moderate quality site (40800) are located within close proximity of each other in the upper left quadrant and display rich and highly similar molluscan communities. The other good quality sites (52700, 39800, 44000) are more scattered. Site 39800 was designated as a good quality site by the VMM in 2007 but since then the water quality appears to have decreased. As we observed, oil seems to be floating on the water surface and several dead birds, mainly pigeons, were found on the river banks. The pollution is probably due to the dismantling of the nearby sugar factory. Twelve snails belonging to five species were found at the site. Site 52700 displays a similar pattern: it was labelled as a good quality site by the VMM in 2007 but the molluscan community is strongly impoverished containing only eight individuals belonging to four species. The site is an isolated old arm of the Moervaart but isolation alone cannot explain the poor community since the nearby located site 52800, which is also an isolated old arm of the Moervaart, contains a much richer community. The unnatural concrete right bank might have something to do with the poor community since few molluscs prefer hard substrates. Site 44000 was the most species rich sampling site: in total thirteen species were found. The site is located at the edge of the CA plot and in close proximity of the poorly represented Pisidium subtruncatum and Physa fontinalis (Fig. 8, chapter 5). Possible explanations for their remote positioning are: 1) they prefer extreme environmental conditions, exemplified by Physa fontinalis being very sensitive to pollution (Mouthon & Charvet, 1999); or 2) they accidentally happen to be present at a location with extreme environmental conditions (van Katwijk & ter Braak, 2003).

The moderate quality site 53300 and the bad quality site 53500 are characterised by a poor water quality and high levels of eutrophication. Site 53300 contained mostly Physella acuta, a pollution resistant species, but interpretation of the site needs to be done with caution since the site had been dredged only recently. The poor quality site 53500 contained only one individual of Physella acuta. Shells of other species were discovered but no other species were found alive in the highly polluted water.

As we already mentioned, the purpose of this analysis was to check what effect the water quality has on molluscan communities and whether molluscs are useful indicators of water quality. Since our observations were limited and not always reliable due to changes in water quality since 2007, a comparison was made with the literature available on the subject. Not much is known about the use of molluscs as pollution indicators. The sensitivity of molluscs to eleven water physico‐chemical variables was tested by Mouthon (1996). After investigation he refined his initial list to a list of six environmental parameters and created a pollution tolerance ranking for molluscan species, genera and families based on his environmental parameters and 48 species (Mouthon & Charvet, 1999; Appendix 11). The environmental variables chosen were dissolved oxygen, the biological demand for oxygen, ammonia, nitrites, Kjeldhal’s nitrogen and orthophosphate. These parameters were tested in our study and one of them, nitrates, has significant effect on the molluscan community. From the twenty two species found four are pollution sensitive species (Anisus vortex, Bithynia leachii, Lymnaea stagnalis, Physa fontinalis), three are intermediate species (Dreissena polymorpha, Sphaerium corneum, Valvata piscinalis) and twelve are pollution tolerant species (Acroloxus lacustris, Bathyomphalus contortus, Bithynia tentaculata, Gyraulus albus, Gyraulus crista, Hippeutis complanatus, Musculium lacustre, Physella acuta, Pisidium subtruncatum, Radix balthica, Valvata cristata) according to the ranking. The three planorbid species could not be attributed to one of these classes because they are not included in the ranking by Mouthon & Charvet (1999). Pollution sensitive species were found only in good quality sites, except for Anisus vortex which was also present in moderate quality sites. Physella acuta, a highly pollution tolerant species, dominated moderate and poor quality waters.

6.2 The fossil malacofauna

The following interpretation of the molluscan assemblage assumes that the shells found were deposited where the animals lived. Since doublets of bivalves were regularly found this assumption is no doubt met.

6.2.1 Hypothetical scenarios of the Moervaartdepression development

The lack of radiocarbon dates and other proxy data left us with no other option than to formulate three hypotheses. Both sequences display more or less the same pattern, but the formulation and evaluation of the scenarios will be mainly based on sequence S2, the sequence with the highest resolution (Fig. 23).

Scenario 1

The first scenario dates from the Oldest Dryas/Pleniglacial to the Preboreal. The climate gradually ameliorates from the bottom of the sequence till the first occurrence of Bithynia tentaculata (93 cm depth, sample 13). The arrival of this frost sensitive species coincides with the start of a warmer period, the Bølling, which ends with the change in deposits from lake marl to organic gyttja and/or peat (73 cm depth). The depositional change corresponds with a general climate deterioration marked by two steep declines in temperature alternating in one short warmer phase. The first drop in temperature (63 cm depth) relates to the Older Dryas, the second (52 cm depth) to the Younger Dryas. The intermediate warmer phase (59 cm depth) can be attributed to the Allerød. When the deposits change again (52 cm depth), this time from organic gyttja and/or peat to lake marl, the climate reaches a new warm optimum which we attribute to the Preboreal. One last drop in temperature (9 cm depth) is observed at the top of the lake marl deposits which might indicate the Preboreal Oscillation.

Scenario 2

The second scenario also dates from the Oldest Dryas/Pleniglacial to the Preboreal. The interpretation for the Oldest Dryas/Pleniglacial and the Preboreal is the same as for scenario one, but the deposits in‐between are interpreted differently. The first climate optimum between 93 cm and 73 cm is attributed to the Bølling/Allerød. The absence of a distinctive Older Dryas in‐between the Bølling and the Allerød has been observed elsewhere and because of this some authors question the existence of the Older Dryas (Björck, 1984). Consequently the organic deposits between 73 and 52 cm depth are assigned to the Younger Dryas.

Fig. 23: The different scenarios of the Moervaartdepression development as displayed by the temperature index (C: cold, W: warm).

Scenario 3

Scenario three spans the shortest time, that is, from the Oldest Dryas/Pleniglacial to the Allerød. Similar to scenario one, the first two phases are assigned to the Oldest Dryas/Pleniglacial and the Bølling. The organic deposits between 73 and 52 cm depth correspond to the Older Dryas and the upper lake marl deposits indicate the Allerød with an Intra Allerød Cold Period at the top of the sequence (Yu & Eicher, 2002).

Scenario two is probably the most likely one. The absence of the Older Dryas in this scenario has been observed elsewhere, as well as an unstable climate with multiple oscillations in the Younger Dryas. The first scenario is also possible, although it is strange to find a steeper decrease in temperature for the Older Dryas than for the Younger Dryas. The least likely option is scenario three which indicates a thick sequence of deposits originating from the Older Dryas, a period which lasted only about a thousand years.

6.2.2 Malacological/environmental phases

The evolution of the molluscan communities can be discussed in terms of habitat preferences and palaeoenvironment adopting the second scenario which we prefer. In the discussion the differences between sequence S2 and S4 will be highlighted.

Pleniglacial/Oldest Dryas

Zone A/a (S2: samples 18 to 14, S4: samples 14 to 8) is attributed to the Oldest Dryas and/or the Pleniglacial. The sand deposits at the bottom of sequence S4 (sample 14) contain only three highly fragmented individuals of Gyraulus crista. The small number of shells and the poor preservation raise some doubt about the origin of these specimens. According to Verbruggen (1971), the sand deposits contain pre‐Weichselian pollen, but it is not clear whether the same can be said for the molluscs. Theoretically, they may have been added to the sands from younger deposits through bioturbation, but there are no indication of bioturbation. In sample 18 of sequence S2 the terrestrial species Succinea putris was found; it prefers to live along rivers, lakes or ditches and in swamps or moist grasslands. Within the moist terrestrial landscape a shallow, temporary pool or probably a set of pools started to develop. Such periodic drying waters are the preferential habitat of Galba truncatula, an amphibious snail also found in sample 18; it lives both in water and on land but always in moist conditions (Gittenberger & Janssen, 2004). As the lake increased in size (S2: sample 18 and 17, S4: sample 13) several other species entered the lake. Among the first colonisers are Gyraulus albus and Gyraulus crista, two pioneer species. They are such effective dispersalists because they are small and can easily get stuck in animal fur or feathers. The small spines on the shell of Gyraulus crista f. cristata also aid transport (Gittenberger & Janssen, 2004). Along with these two species, Hippeutis complanatus and Radix balthica enter the lake. Hippeutis complanatus in the community indicates the end of periodic desiccations of the lake, since H. complanatus cannot tolerate these conditions. After this first phase (S2: samples 18 and 17, S4: samples 14 and 13), the environmental indices show a gradual increase in temperature and lake size. New species enter the lake from sample 16 (S2) and sample 12 (S4) onwards, namely Valvata piscinalis, Physa fontinalis, Myxas glutina and Lymnaea stagnalis. They indicate well‐vegetated and well oxygenated slow to still waters

(Kerney, 1999). Although Lymnaea stagnalis is not present in large numbers, its presence indicates a minimum temperature of the lake: Lymnaea stagnalis is frost sensitive and freezes at ‐1.2 to ‐2.2°C; below ‐4 to ‐5°C irreparable damage sets in (Gittenberger & Janssen, 2004). A small number of bivalve species was also recovered from the deposits (S2: samples 18 to 14, S4: samples 13 to 8). The observed bivalves are either pioneer species (Mouthon, 1990) or species that prefer well‐vegetated littoral zones (Davies, 2008). The steady increase in biodiversity is also visible in the biodiversity indices. For the Pleniglacial/Oldest Dryas, we can conclude that the environment probably evolved from a moist grassland or swamp, over a (set of) temporary drying pool(s) to a permanent aquatic habitat with vegetation developing in the littoral zone. The gradual increase in species diversity is no doubt linked to habitat diversification; larger lakes generally hold a wider range of habitats (Økland, 1990).

Bølling/Allerød

Zone B/b (S2: samples 13 to 11, S4: samples 7 and 6) is characterised by an abrupt change in the molluscan assemblages. Most species of zone A/a (S2: samples 18 to 14, S4: samples 14 to 8) remain present but the steep increase in the number of shells and the first appearance and abrupt dominance of Bithynia tentaculata reflect a considerable change in the environment. We cannot say with certainty whether the increase in the number of shells represents a true rise in molluscan biomass or whether it is an artefact. If the density of the molluscan population was constant, a lower sedimentation rate would imply increased accumulation of shells; we do not find this likely (Moine et al., 2008). According to the environmental indices the community change was linked to a steep increase in temperature and lake size. Molluscan species with a preference for warm climates dominate the community. Lymnaea stagnalis and Bithynia tentaculata are such species as they cannot survive temperatures below ‐4 to ‐5°C. Valvata cristata enters the lake in the middle of zone B/b (S2: sample 12, S4: sample 6) and indicates well‐vegetated and well oxygenated conditions (Kerney, 1999). The low abundance of freshwater bivalves is more difficult to explain. According to Mouthon (1990) Pisidium and Sphaerium species are typically present in the littoral zone of lakes. Since the lake was large at the time, it is possible that we overlooked part of the molluscan community by sampling only the deepest part of the lake. In the future this sampling bias can be overcome by taking a series of samples from the deepest part of the lake to the presumed edge of the lake. The absence of a distinct climate cooling during Older Dryas can be an artefact linked to the coarse sampling, but the actual existence of the Older Dryas is also doubted by some authors (e.g. Björck, 1984). For zone B/b, we can conclude that the Moervaartdepression was characterised by a large well‐vegetated and well oxygenated lake. The water body was either stagnant or slow moving during the Bølling/Allerød, since few indicators for water movement were found.

Younger Dryas

Zone CDEF/cde (S2: samples 10 to 7, S4: samples 5 and 3) shows a major change in depositional environment and in molluscan community. The laminated marl which dominated the Bølling/Allerød deposits (S2: samples 13 to 11, S4: samples 7 and 6) is replaced by organic peaty deposits with sandy intercalations. The sands are probably aeolian and were blown into the lake during cold spells. The environmental indices point towards a steep decrease in temperature and lake size. Owing to these observation zone CDEF/cde was attributed to the Younger Dryas. The presence of two marked

68 declines in temperature, separated by a warmer phase, is striking but climatic instability during the Younger Dryas has been observed before (Ebbesen & Hald, 2004). Despite the climate deterioration the biodiversity indices increase due to the return of several Pisidium species. Their return might be linked to the decrease in temperature as Pisidium species need well oxygenated waters and cold waters generally contain more oxygen. Also, the shrinking of the lake may have moved littoral habitats towards the lake centre. The climate cooling is best demonstrated by the steep decrease in the frost sensitive species (Bithynia tentaculata and Lymnaea stagnalis) and the simultaneous increase in the frost resistant species (Valvata piscinalis and Pisidium obtusale). Pisidium obtusale freezes at ‐11.1°C and Valvata piscinalis becomes active and starts reproducing already at 5°C (Gittenberger & Janssen, 2004). The first drop in temperature (S2: sample 9, S4: sample 5) affects mainly Bithynia tentaculata. The second decline (S2: sample 7, S4: sample 3), although less severe, eliminates more species from the lake. According to Kerney (1999) the presence of Pisdium obtusale indicates not only cold conditions but also a poor environment. The gradual decrease in Gyraulus crista and Radix balthica, and the absence of Valvata cristata can be linked to the progressive shallowing of the lake, but the lake probably still had a depth of more than one meter, since Valvata piscinalis remained abundant (Bennike et al., 1998; Jones et al., 2000). The environmental indices display an increase in the amount of water movement during the cold phases. Melt water possibly flowed through the shallow lake during spring. However, the absence of large freshwater mussels such as unionids suggests that the Moervaartdepression was a closed water body. The distribution of unionids depends on fishes since juvenile unionids parasitize on them (Gittenberger & Janssen, 2004). If the lake was an open system linked to a river we would expect to find unionids brought in together with fishes, but no unionids and no fish bones were found.

In between the cold peaks the climate ameliorated for a short period (S2: sample 8, S4: sample 4). The observed rise in temperature is accompanied with a rise in the lake size and a stagnation of the lake water. In general the composition of the molluscan community, the values of the biodiversity indices and the environmental indices resemble the Bølling/Allerød. The biggest difference is observed in absolute abundance: samples of zone E/e contain less than 100 individuals which is more than ten times less than the samples of Bølling/Allerød. This decrease in absolute abundance is seen for the whole of the Younger Dryas as opposed to the Bølling/Allerød and may be linked to climate deterioration. Poor preservation of shells in peaty deposits is another explanation. Peat is an acidic deposit which can dissolve the shells. Moreover it is affected by compaction which may crush the shells. The definition of the end of the Younger Dryas is less straightforward. According to sequence S2 the climate ameliorates at the transition from the organic, peaty deposits (sample 7, ZF) to the marl (sample 6, ZG). According to sequence S4 the climate ameliorates later (sample 3, Zf). The difference may be explained by lateral variability in the lake. During the Younger Dryas, the molluscan community that dominated the Pleniglacial/Oldest Dryas and the Bølling/Allerød (Gyraulus albus, Gyraulus crista, Hippeutis complanatus, Physa fontinalis, Radix balthica) is replaced by another community with similar habitat preferences (Bithynia tentaculata, Valvata cristata), that is, a preference for well‐vegetated and well oxygenated waters (Kerney, 1999). The high abundance of Myxas glutinosa, Pisidium obtusale and Valvata piscinalis at the end of the Younger Dyras indicates clear, permanent and well oxygenated but poor waters.

Preboreal

Zone GHIJ/f (S2: samples 6 to 1, S4: sample 2) was assigned to the beginning of the Holocene, the Preboreal, since the environmental indices display an increase in temperature and lake size. The samples from the zone are characterised by a low number of shells. In marl deposits shell preserve well and the decline of the number of shells is either linked to a general decrease in molluscan biomass or a change in sedimentation rate. At the beginning of the Preboreal several species that had disappeared or were limited during the Younger Dryas reappear or increase. Examples are Gyraulus albus, Physa fontinalis, Radix balthica and Valvata cristata. As in other warm phases the Pisidium species decrease or disappear. Frost sensitive species increase in number, whereas frost resistant species decline. According to the molluscan community and the environmental indices the Preboreal warm phase is interrupted by a steep but short cold phase, the Preboreal Oscillation (S2: sample 3, ZI). During this short climate deterioration frost resistant species (Pisidium obtusale, Valvata piscinalis) and Pisidium species increase at the cost of frost sensitive species. The environmental indices also display an increase in the amount of water movement during the cold phases. As we already mentioned, it is possible that rivers filled with melt water flowed through the shallow lake during spring. The Preboreal Oscillation has been observed in European lake records, in marine records and in the ice core δ180 data (Noe‐Nygaard & Heiberg, 2001) and occurred in the entire North Atlantic region (Björck et al., 1997). After the Preboreal Oscillation (S2: sample 2 and 1) the molluscan community does not recover: the community contains only pioneer species (Gyraulus crista, Sphaerium corneum), Bithynia tentaculata and Valvata cristata. The decrease in species richness is indicated by all the diversity indices except evenness, because sample 2 contains only one Bithynia tentaculata and one Gyraulus crista. The modern soil (S4: sample 1, Zg) is characterised by the transition from an aquatic to a terrestrial environment. The transition was probably gradual since both Galba truncatula and Valvata cristata were recovered and these species can survive temporary desiccation of their habitat. The terrestrial community contains six species, namely Carychium minimum, Punctum pygmaeaum, Pupilla muscorum, Succinella oblonga, Vertigo sp. and Vallonia pulchella. As we already mentioned, we were not able to identify the Vertigo sp. to species, but based on the general form of the shell and the habitat preferences of the other terrestrial species the Vertigo sp. possibly belongs to Vertigo antivertigo. The presence of Pupilla muscorum may be indicative of nearby dry grassland. For the Preboreal, we can conclude that the gradual re‐ establishment of the community was interrupted by the Preboreal Cold Oscillation after which the molluscan community did not recover and the lake disappeared. The last aquatic fauna was characteristic of well‐vegetated and well oxygenated waters. The terrestrial fauna indicates open marshland or damp grassland and nearby dry grassland.

6.3 Comparison with molluscan evidence of elsewhere

The formation of lakes in depressions at the end of the Last Glacial period is a widespread phenomenon. Most lakes were relatively shallow, short‐lived and sensitive to the Late Glacial climatic oscillations (Davies, 2004). They often contain a wide variety of biological material as was the case for the Moervaartdepression. Davies (ibid.) states that molluscs recovered from lakes have done much to reveal the lake histories and aspects of molluscan biogeography. Therefore, a comparison was made with some molluscan studies from neighbouring countries and northern European lowland regions.

Molluscan studies from France (e.g. Limondin & Rousseau, 1999; Limondin, 1995; Limondin‐Lozouet & Preece, 2004; Rousseau & Puissegur, 1999) and Germany (e.g. Meyrick, 2001, 2003; Moine et al., 2008) were of limited use since they focus mainly on terrestrial species. If freshwater molluscs were recovered from the study sites, they were often only listed in a table or clustered as “aquatics” after which at least limited interpretation was given. One study can be mentioned, that is, a climate history reconstruction bases on land snail assemblages by Rousseau et al. (1998). In the study temperatures were estimated based on recent land snails: for the Younger Dryas the estimates vary between ‐6°C and ‐5°C (± 2.94°C) in winter and 15.5°C and 16.5°C (± 2.8°C) in summer for the coldest and warmest months respectively. The winter estimates explain why Bithynia tentaculata, which can only tolerate temperatures down to ‐4°C, steeply declines during cold events whereas Valvata piscinalis and Pisidium obtusale, which freeze only at ‐11°C, increase.

Studies from Northern England (e.g. Jones et al., 2000; Keen et al., 1984; Keen et al., 1988; Sparks, 1962) and Wales (Walker et al., 1993) give more extensive analyses of molluscan faunas. Detailed sequences of the Late Glacial and the Holocene were recovered from lake marls at several sites. In general the communities are dominated by Gyraulus crista, Gyraulus laevis, Lymnaea peregra, Valvata cristata and Valvata piscinalis during warm phases, whereas deposits from cold phases often contain no molluscs at all. Bithynia tentaculata, which makes up a large part of the Moervaartdepression assemblages, is observed only in lakes from the Holocene onwards. The poor and often monotonous communities in England differ substantially from our molluscan sequence. This difference may be explained by the difference in latitude.

Hoek et al. (1999) studied the molluscan succession in gyttja deposits from the southern Netherlands. The sequence covers a time period from the Bølling to the Younger Dryas. Bithynia tentaculata dominates the community in the Allerød and reaches high abundances soon after its first appearance. In the next phase Bithynia tentaculata steeply declines and is replaced by Valvata piscinalis and Pisidium species. The change from a Bithynia tentaculata dominated community to a Valvata piscinalis dominated community points to colder climate conditions. A similar pattern was observed in our sequence which is quite understandable: the Netherlands are our nearest neighbouring country and have a similar climate and topography. Another study from the Netherlands by Meijer (1970) from Holocene marl deposits gives only a list of species.

The sequence that resembles our data most comes from Lake Tøvelde in Denmark (Noe‐Nygaard & Heiberg, 2001). The lacustrine deposits date from the Oldest Dryas to the Boreal and were taken from the deepest part of the former lake. Unfortunately the molluscan sequence covers only the Allerød to the Younger Dryas. The early formation of Lake Tøvelde is indicated by the transition from terrestrial molluscs to freshwater molluscs at the beginning of the Allerød. The early molluscan community is composed of Gyraulus crista, Gyraulus laevis, Lymnaea peregra, Physa fontinalis and Pisidium species, and later Valvata piscinalis is added. At the end of the Allerød the community is composed of Pisidium species, Sphaerium species, Valvata piscinalis, Valvata cristata, Lymnaea peregra and Gyraulus laevis. The community stays more or less the same at the beginning of the Younger Dryas but the diversity decreases clearly as the Younger Dryas progresses. Although the development of Lake Tøvelde starts later, the composition and succession of the early molluscan community largely resembles that of the Moervaartdepression. For both lakes the molluscan community indicates the development from a shallow, near‐shore lake to a deeper lake. The major

71 difference between the sequences is the absence of Bithynia tentaculata in Lake Tøvelde, which is probably linked to the difference in latitude. A second dissimilarity is linked to the evolution of the lake level. The molluscs from the Moervaartdepression indicate a larger lake size during warm phases and the opposite during cold periods. Lake Tøvelde on the contrary displays a very high lake level during the Younger Dryas. This lake level rise was probably caused by a rise in groundwater level in the Baltic Basin caused by the damming of the Baltic Ice Lake. As opposed to the general trend in England Lake Tøvelde did support molluscan communities during the cold periods even though both locations are situated at similar latitude. The lake level rise during the Younger Dryas may have prevented freezing of the lake to the bottom thereby sustaining the molluscan community. Bithynia tentaculata was detected by Bennike et al. (1998) in lake marl from the south western Baltic Sea but these deposits were dated to the early Holocene.

Molluscan studies on lacustrine deposits have also been performed in Poland. Alexandrowicz & Nowaczyk (1982) studied Late Glacial and early Postglacial lake deposits which demonstrate several similarities with the Moervaartdepression assemblages: the Allerød deposits displayed a community typical of larger water basins and contained Bithynia tentaculata, whereas the deposits from the Younger Dryas represent a progressive shallowing of the lake and a dominance of Pisidium species and Valvata piscinalis. The presence of Bithynia tentaculata in the Allerød deposits is striking, as none of the other studies cited detected Bithynia tentaculata in the Allerød. Another sequence of Late Glacial and Holocene lacustrine sediments from central Poland (Apolinarska & Ciszewska, 2006; Apolinarska, 2009) also display similarities to the Moervaartdepression: Pisidium species are especially abundant during the Younger Dryas and the Preboreal Oscillation and species diversity was lower during warm than cold phases. Mostly the opposite is observed, that is, poor communities during cold phases and rich communities during warm phases. For our sequence and the Polish sequence, the observed increase in species richness is linked to an increase in Pisidium species and this can be explained in various ways as we already mentioned earlier. The first explanation is linked to animal physiology: Pisidium species require well oxygenated waters and cooler water generally contains more oxygen. The second is linked to the lake level: shallowing of the water basin causes the littoral zone to shift towards the centre of the lake; Pisidium species prefer to live in the littoral zone (Mouthon, 1990). We note that Apolinarska & Ciszewska (ibid.) observed a clear relationship between the size of molluscan shells and temperature. At relatively low temperatures shells are small and thin, whereas large and thick shells indicate a warmer climate. A similar pattern was observed for the malacofauna of the Moervaartdepression: large shell size was especially exhibited during the Bølling/Allerød.

6.4 Comparison of the fossil and recent malacofauna

The fossil and recent malacofauna resemble each other. Since the Late Glacial some species have disappeared while others appeared or were newly introduced by man. Amongst the lost species are several Pisidium species and the glutinous snail Myxas glutinosa. The only Pisidium species recovered from the recent sampling sites was Pisidium subtruncatum, a generalist species. The decline and disappearance of Myxas glutinosa is a widespread phenomenon and is attributed to several causes. Myxas glutinosa prefers clear, stagnant waters with abundant aquatic vegetation, especially dense Stratiotes aloides vegetation. These biotopes have become very scarce nowadays due to local

72 pollution and clearing of ditches (Boesveld et al., 2009b). Gittenberger & Janssen (2004) however question the decline to some degree and attributed it to a decrease in sightings, that is, the snails are easily overlooked because they are less recognisable as snail once they are taken out of the water. Six species in total were recovered only from the recent malacofauna, among which two introduced and possibly invasive species. Dreissena polymorpha is one of those newly introduced species. Since the end of the eighteenth century it has spread throughout Europe from areas around the Black and Caspian Sea (Gittenberger & Janssen, 2004). The first sighting in Belgium dates back to 1869 (Adam, 1960). Dreissena polymorpha is a good coloniser and has become a threat to some indigenous bivalves. Adult unionids live partially buried in the sediment with their posterior shell exposed to the water column. The exposed shell is often colonised by Dreissena polymorpha individuals that attach themselves to the shell by means of byssal fibers. Shell infestation is believed to impair the metabolic activity of the unionids and can lead to depletion of the energy reserves and eventually starvation to death (Ricciardi et al., 1998). Dreissena polymorpha was only recovered from one location in the Zuidlede attached to rocks. It has probably been introduced there by boats from the nearby yacht harbour. The second introduced species is Physella acuta. As opposed to Dreissena polymorpha the origin of Physella acuta is still debated. Early hypotheses pose that Physella acuta is native to Europe, however, Dillon et al. (2002) concluded that it may have originated in northern America and has since then become invasive on other continents. The worldwide distribution is probably partially to blame on the aquarium trade (Appleton, 2003) along with the snails’ abilities to quickly colonise new habitats and to move upstream (de Kock & Wolmarans, 2007). The first sighting in Belgium dates back to 1869 (Adam, 1960). Physella acuta is commonly known as the ‘sewage snail’ (de Kock & Wolmarans, 2007) as it was mainly recovered from polluted water basins. The three other species recovered only from our recent sampling sites are species that have been reported from other Quaternary deposits. The absence of Bithynia leachii is not particularly strange because sightings from pre‐Holocene deposits are not frequent (Gittenberger & Janssen, 2004).

Conclusions 7

Investigation of eleven recent sampling sites within the catchment of the Moervaart and Zuidlede River displayed that water quality does influence the molluscan community. When the water quality decrease the number of molluscs and species decrease as well. The environmental parameters that underlie this pattern are biological water quality, the amount of nitrates, chloride and chemical oxygen demand. Pollution has led to the disappearance of at least one species, Myxas glutinosa. The disappearance of all Pisidium species but one, Pisidium subtruncatum, may be linked to pollution but other causes are not excluded. Five species were observed for the first time in the recent samples: three of these species (Bithynia leachii, Lymnaea palustris, Musculium lacustre) have been recovered from Quaternary deposits elsewhere; the others (Dreissena polymorpha, Physella acuta) were introduced by man.

The malacological analysis based on sequence S2 and S4 shows that the palaeoenvironment of the Moervaartdepression underwent major changes during the Late Glacial and the early Holocene. A total of four periods characterised by drastic changes can be defined.

During the Pleniglacial/Oldest Dryas the Moervaartdepression developed from a terrestrial into an aquatic environment via a (set of) small temporary pool(s). The early lake was mainly inhabited by typical pioneer species (Gyraulus albus, Gyraulus crista and Pisidium species) but as the lake size increased, due to a rise in the water table and fluvial supply, new species entered. Habitat diversification probably created an increase in species diversity since larger lakes generally hold a wider range of habitats. By the end of the Pleniglacial/Oldest Dryas the lake had developed into a large well‐vegetated and well oxygenated stagnant water body.

The Bølling/Allerød represents a climate optimum during which the Moervaartdepression was represented by a large well‐vegetated and well oxygenated stagnant lake. The presence of adult individuals of several drought sensitive species indicates that the lake did not dry out during summer. The exceptionally high molluscan biomass characterises the Bølling/Allerød although we cannot say with certainty that this is not an artefact created by changes in sedimentation rate.

The Younger Dryas displays a major climate deterioration consisting of two steep declines in temperature separated by a short climate amelioration. During this period, the molluscan community of the Pleniglacial/Oldest Dryas and Bølling/Allerød (Gyraulus albus, Gyraulus crista, Hippeutis complanatus, Physa fontinalis, Radix balthica) was replaced by a new community with similar habitat preferences (Bithynia tentaculata, Valvata cristata), that is, a preference for well‐vegetated and well oxygenated waters. Some samples of the Younger Dryas contained low numbers of shells which may

74 be due to a decrease in molluscan biomass linked to the climate deterioration or due to the poorer preservation of shells in peaty deposits.

During the Preboreal the climate ameliorates and the molluscan community begins to recover from the foregoing cold period, but the amelioration is only short‐lived as a new cold phase interrupts the re‐establishment, after which the community does not recover. The lake dried out due to a lowering of the water table. The recovered terrestrial fauna indicates open marshland or damp grassland and nearby dry grassland.

In general we can conclude that freshwater molluscs proved to be good indicators for the reconstruction of the palaeoenvironment. The comparison of the fossil and recent fauna indicated that molluscs are quite resistant to human‐induced habitat alterations. The investigation of the recent fauna showed that molluscs may be useful indicators for water quality assessment, although further investigation on the subject is necessary.

Suggestions for further investigation follow. Further analyses with higher resolution of the transition from the Younger Dryas and the Preboreal can trace the exact position of this transition. The creation of a transect of samples from the middle of the lake to the banks would help to understand better the distribution of the Pisidium species. High resolution investigation of the first entrance of Bithynia tentaculata may find out whether the species increases gradually in abundance or whether it behaves as an invasive species. Further investigation of the relation between biodegradable pollution and the composition of the molluscan community may help to set up a model to determine water quality based on molluscs alone.

References O

Adam, W. (1960) Faune de Belgique, Mollusques I, Mollusques terrestres et dulcicoles. Brussels: Royal Belgian Institute of Natural Sciences.

Alexandrowicz, S.W. & Nowaczyk, B. (1982) Late‐glacial and Holocene lake sediments at Pomorsko near Sulechów. Quaestiones Geographicae 8, 5‐17.

Anderson R. (2005) An annotated list of the non‐marine mollusca of Britain and Ireland. Journal of Conchology 53(6), 607‐637.

Apolinarska, K. & Ciszewska, M. (2006) Late Glacial and Holocene lacustrine molluscs from Wielkopolska (central Poland) and their environmental significance. Acta Geologica Polonica 56(1), 51‐66.

Apolinarska, K. (2009) δ18O and δ13C isotope investigation of the Late Glacial and early Holocene biogenic carbonates from the Lake Lednica sediments, western Poland. Acta Geologica Polonica 59(1), 111‐121.

Appleton, C.C. (2003) Alien and invasive fresh water Gastropoda in South Africa. African Journal of Aquatic Science 28, 69‐81.

Bates, C.D., Coxon, P., Gibbard, P.L. (1978) A new method for the preparation of clay‐rich sediment samples for palynological investigation. New Phytologist 81, 459‐463.

Bennike, O., Lemke, W., Jensen, J.B. (1998) Fauna and flora in submarine early‐Holocene lake‐marl deposits from the southwestern Balthic Sea. The Holocene 8(3), 353‐358.

Björck, S. (1984) Bio‐ and chronostratigraphic significance of the Older Dryas Chronozone – on the basis of new radiocarbon dates. Journal of the Geological Society of Sweden 106(1), 81‐91.

Björck, S., Rundgren, M., Ingolfsson, O., Funder, S. (1997) The Preboreal Oscillation around the Nordic Seas: terrestrial and lacustrine responses. Journal of Quaternary Science 12, 455‐465.

Boesveld, A., Gmelig Meyling, A.W., de Bruyne, R.H. (2009a) Slakken van de Habitatrichtlijn waarnemen. Heemstede: Stichting Anemoon.

Boesveld, A., Gmelig Meyling, A.W., van Lente, I. (2009b) Inhaalsslag Verspreidingsonderzoek. Mollusken van de Europese Habitatrichtlijn. Resultaten van het inventarisatiejaar 2008. Platte schijfhoren Anisus vorticulus. Heemstede: Stichting Anemoon. Briers, R.A. (2003) Range size and environmental calcium requirements of British freshwater gastropods. Global Ecology & Biogeography 12, 47‐51.

Brönmark, C. (1988) Effects of vertebrate predation on freshwater gastropods: an exclosure experiment. Hydrobiologia 169, 363‐370.

Brönmark, C. & Weisner, S.E.B. (1996) Decoupling of cascading trophic interactions in a freshwater, benthic food chain. Oecologia 108, 534‐541.

Broothaers, L. (1995) Geologie van Vlaanderen, een schets. Brussels: Ministry of the Flemish Community, Department of Natural Resources and Energy.

Crombé, Ph. (ed) (2005) The Last Hunter‐Gatherer‐Fishermen in Sandy Flanders (NW Belgium): The Verrebroek and Doel Excavation Projects, Part 1: palaeo‐environment, chronology and features. Ghent: Archaeological Reports Ghent University 3.

Crombé, Ph., Verbruggen, C. (2002) The Lateglacial and early Postglacial occupation of northern Belgium: the evidence from Sandy Flanders, in: Eriksen, B.V., Bratlund, B. (eds) Recent studies in the Final Paleolithic of the European Plain, Proceedings of a UISPP Symposium, Stockholm, 14‐17. October 1999 (=Jutland Archaeological Society Publications), Stockholm, 165‐180.

Davies, P. (2008) Snails. Archaeology and Landscape Change. Oxford: Oxbow Books. de Bruyne, R.H., Gmelig Meyling, A.W., Boesveld, A. (2008) Kleverige poelslak Myxas glutinosa Draparnaud, 1805. In: Kalkman, V.J. (ed) De soorten van het leefgebiedenbeleid. Leiden: EIS Nederland, 116‐120. de Kock, K.N. & Wolmarans, C.T. (2007) Distribution and habitats of the alien invader freshwater snail Physa acuta in South Africa. Water SA 33(5), 717‐722.

De Moor, G. (1963) Bijdrage tot de kennis van de fysische landschapsvorming in Binnen‐Vlaanderen. Tijdschrift van de Belgische Vereniging voor Aardrijkskundige Studies 32, 329‐443.

De Moor, G. (1995) De zanden van de Vlaamse Vallei. In: Gullentops, F. (ed) Grondstoffen in Vlaanderen. Brussel: Ministerie van de Vlaamse Gemeenschap.

De Moor, G. & Heyse, I. (1978) De Morfologische evolutie van de Vlaamse Vallei. De Aardrijkskunde 4, 343‐375.

De Moor, G. & Heyse, I. (1994) Geomorfologische Kaart, Kaartblad Lokeren. Brussel: Ministerie van de Vlaamse Gemeenschap.

Devriese, R., Warmoes, T., Vercoutere, B. (1992) Land‐ en zoetwatermollusken van de Benelux. Gent: Jeugdbond voor Natuurstudie en Milieubescherming.

Dillon, R.T.(Jr.), Wethington, A.R., Rhett, J.M., Smith, T.P. (2002) Populations of the European freshwater pulmonate Physa acuta are not reproductively isolated from American Physa heterostropha or Physa integra. Invertebrate Biology 12(3), 226‐234.

Ebbesen, H. & Hald, M. (2004) Unstable Younger Dryas climate in the northeast North Atlantic. Geology 32, 673‐676.

Gittenberger, E. & Janssen, A.W. (eds) (2004) De Nederlandse zoetwatermollusken. Recente en fossiele weekdieren uit zoet en brak water. – Nederlandse Fauna 2. Leiden: Nationaal Natuurhistorisch Museum Naturalis, KNNV Uitgeverij & EIS‐Nederland.

Gloër, P. & Meier‐Brook, C. (1994) Süsswassermollusken. Ein Bestimmungsschlüssel für die Bundesrepublik Deutschland, eleventh edition. Hamburg: Deutsche Jugendbund für Naturbeobachtung.

Goslar, T., Arnold, M., Tisnerat‐Laborde, N., Czernik, J., Więckowski, K. (2000) Variations of Younger Dryas atmospheric radiocarbon explicable without ocean circulation changes, Nature 403, 877‐880.

Hammer, Ø. & Harper, D.A.T. (2006) Paleontological data analysis. Oxford: Blackwell.

Hammer, Ø., Harper, D.A.T., Ryan. P.D. (2001) PAST: Paleontological statistics software package for education and data analysis. Palaeontologia Electronica 4, 1‐9.

Hays, J.D., Imbrie, J., Shackleton, N.J. (1976) Variations in the Earth’s Orbit: Pacemaker of the Ice Ages, Science 194(4270), 1121‐1132.

Heynderickx, M. (1982) Genese en Evolutie van de Moervaartdepressie. Ghent University: Unpublished Master thesis.

Heyse, I. (1979) Bijdrage tot de geomorfologische kennis van het noordwesten van Oost‐Vlaanderen (België). Brussels: Royal Flemish Academy of Belgium for Science and the Arts.

Hill, M.O. (1979) DECORANA – A FORTRAN program for Detrended Correspondence Analysis and Reciprocal Averaging. New York: Cornell University, Sections of Ecology and Systematics.

Hoek, W.Z., Bohncke, S.J.P., Ganssen, G.M., Meijer, T. (1999) Lateglacial environmental changes recorded in calcareous gyttja deposits at Gulickshof, southern Netherlands. Boreas 28(3), 416‐432.

Jones, R.L., Keen, D.H., Robinson, J.E. (2000) Devensian lateglacial and early Holocene floral and faunal records from N.E. Northumberland. Proceedings of the Yorkshire Geological Society 53(2), 97‐ 110.

Jongepier, I. (2009) Historisch‐geografisch onderzoek van de Moervaartdepressie en omgeving, Een studie aan de hand van historische kaarten tussen de 13de en 18de eeuw en literatuur. Ghent University: Unpublished Master thesis.

Jongman, R.H.G., ter Braak, C.J.F., van Tongeren, O.F.R. (1995) Data analysis in community and landscape ecology. Cambridge, Cambridge University Press.

Kasse, C. (2002) Sandy aeolian deposits and environments and their relation to climate during the Last Glacial Maximum and the Lateglacial in northwest and central Europe. Progress in Physical Geography 26(4), 507‐532.

Keen, D.H., Jones, R.L., Evans, R.A., Robinson, J.E. (1988) Faunal and floral assemblages from Bingley Bog, west Yorkshire, and their significance for Late Devensian and Early Flandrian environmental changes. Proceedings of the Yorkshire Geological Society 47, 125‐138.

Keen, D.H., Jones, R.L., Robinson, J.E. (1984) A Late Devensian and early Flandrian fauna and flora from Kildale, North‐east Yorkshire. Proceedings of the Yorkshire Geological Society 44, 385‐397.

Kerney, M.P. & Cameron, R.A.D. (1980) Elsevier slakkengids. Met meer dan 1000 afbeeldingen waarvan 400 in kleur. Amsterdam: Elsevier.

Kerney, M.P. (1999) Atlas of the land and freshwater molluscs of Britain and Ireland. Colchester: Harley.

Kiden, P. (1991) The Lateglacial and Holocene Evolution of the Middle and Lower River Scheldt, Belgium. In: Starkel, L., Gregory, K.J., Thornes, J.B. (eds) Temperate Palaeohydrology. Chichester: Wiley, 283‐299.

Kiden, P. (2006) De evolutie van de Beneden‐Schelde in België en Zuidwest‐Nederland na de laatste ijstijd. Belgeo 3, 279‐294.

Legendre, P. & Legendre, L. (1998) Numerical Ecology. Amsterdam: Elsevier Science.

Limondin, N. & Rousseau, D.‐D. (1999) Holocene climate as reflected by a malacological sequence at Verrieres, France. Boreas 20, 207‐229.

Limondin, N. (1995) Late Glacial and Holocene Malacofaunas from Archaeological Sites in the Somme Valley (North France). Journal of Archaeological Science 22, 683‐698.

Limondin‐Lozouet, N. & Preece, R.C. (2004) Molluscan successions from the Holocene tufa of St Germain‐le‐Vasson, Normandy (France) and their biogeographical significance. Journal of Quaternary Science 19, 55‐71.

Meijer, T. (1970) Het Blekkinkveen bij Winterswijk. Mededelingen van de Werkgroep voor Tertiaire en Kwartaire Geologie 7(3), 89‐91.

Meyrick, R.A. (2001) The development of terrestrial mollusc faunas in the ‘Rheinland region’ (western Germany and Luxembourg) during the Lateglacial and Holocene. Quaternary Science Reviews 20, 1667‐1675.

Meyrick, R.A. (2003) Holocene molluscan faunal history and environmental change at Kloster Muhle, Rheinland‐Pfalz, western Germany. Journal of Quaternary Science 18, 121‐132.

Moine, O., Rousseau, D.‐D., Antoine, P. (2008) The impact of Dansgaard‐Oeschger cycles on the loessic environment and malacofauna of Nussloch (Germany) during the Upper Weichselian. Quaternary Research 70, 91‐104.

Mouthon, J. & Charvet, S. (1999) Compared sensitivity of species, genera and families of Mollusca to biodegradable pollution. Annales de Limnologie 35(1), 31‐39.

Mouthon, J. (1990) The importance of climatic conditions for the structure of mollusc communities in European lakes. Archiv für Hydrobiologie 118(3), 353‐370.

Mouthon, J. (1996) Molluscs and biodegradable pollution in rivers: Studies into the limiting values of 11 physico‐chemical variables. Hydrobiologia 319(1), 57‐63.

Noe‐Nygaard, N. & Heiberg, E.O. (2001) Lake‐level changes in the Late Weichselian Lake Tøvelde, Møn, Denmark: induced by changes in climate and base level. Palaeogeography, Palaeoclimatology, Palaeoecology 174, 351‐382.

Økland J. (1990) Lakes and snails. Environment and Gastropoda in 1,500 Norwegian lakes, ponds and rivers. Oegstgeest: Universal Book Services/W. Backhuys.

Oksanen, J. (2010) Multivariate analysis of Ecological Communities in R: vegan tutorial. March 11 2010. (http://vegan.r‐forge.r‐project.org/, 22‐05‐’10)

Oksanen, J., Kindt, R., Legendre, P., O’Hara, B., Simpson, G.L., Stevens, M.H.H., Wagner, H. (2008) Vegan: community ecology package, Version R, package version 1.13‐1.

Ricciardi, A., Neves, R.J., Rasmussen, J.B. (1998) Impending extinctions of North American freshwater mussels (Unionoida) following the zebra mussel (Dreissena polymorpha) invasion. Journal of Animal Ecology 67, 613‐619.

Rousseau, D.‐D. & Puissegur, J.J. (1999) Climatic interpretation of terrestrial malacofaunas of the last interglacial in southeastern France. Palaeogeography Palaeoclimatology Palaeoecology 151, 321‐ 336.

Rousseau, D.‐D., Preece, R., Limondin‐Lozouet, N. (1998) British late glacial and Holocene climatic history reconstructed from land snail assemblages. Geology 26(7) 651‐654.

Sparks, B.W. (1962) The ecological interpretation of Quaternary non‐marine Mollusca. Proceedings of the Linnean Society of London 172, 71‐81.

Spiers (1986) Mineralogisch en petrografisch onderzoek van de kalkrijke sedimenten van de Moervaartdepressie. Ghent University, Unpublished Master thesis.

Stuiver, M., van der Plicht, J. (eds) (1998) INTCAL98: calibration issue. Radiocarbon 40(3), 1041‐1159.

Van Vlaenderen, L., Sergant, J., De Bock, H., De Meireleir, M. (2006) Steentijdvondsten in de Moervaartdepressie (Oost‐Vlaanderen, België): inventaris en geografische analyse. Ghent, Archaeological Inventory Flanders.

Van Zeir, A. (2009) Quartairgeologie en geomorfologie in de omgeving van Kaprijke. Ghent University, Unpublished Master thesis.

Verbruggen, C. (1971) Postglaciale landschapsgeschiedenis van zandig Vlaanderen. Botanische, ecologische en morfologische aspekten op basis van palynologisch onderzoek. Ghent University: Unpublished PhD thesis.

Verbruggen, C. (1979) Vegetational and palaeoecological history of the Late Glacial period in Sandy Flanders (Belgium). Acta Universitatis Ouluensis 82 Geol. 3, 133‐142.

Verbruggen, C., Denys, L., Kiden, P. (1991) Palaeo‐ecologische en geomorfologische evolutie van Laag‐ en Midden‐België tijdens het Laat‐Kwartair. De Aardrijkskunde 3, 357‐376.

Walker, M.J.C. (1995) Climatic changes in Europe during the Last Glacial/Interglacial Transition. Quaternary International 28, 63‐76.

Walker, M.J.C., Griffiths, H.I., Ringwood, V., Evans, J.G. (1993) An early Holocene pollen, mollusc and ostracod sequence from the lake marl at Llangorse Lake, South Wales, U.K. The Holocene 3, 138‐149.

Wallbrink, H. (1992) De Unionidae van Lek en Waal. Correspondentieblad van de Nederlandse Malacologische Vereniging 267, 986‐994.

Wallbrink, H. (1993) De Sphaeriidae van Lek en Waal. Correspondentieblad van de Nederlandse Malacologische Vereniging 270, 3‐12.

Yu, Z. & Eicher, U. (2001) Three amphi‐Atlantic century‐scale cold events during the Bølling‐Allerød warm period. Géographie physique et Quaternaire 55(2), 171‐179.

Websites: http://www.changbioscience.com/genetics/shannon.html (13‐04‐‘10) http://cartogis.ugent.be/sandy_flanders/ (27‐04‐’10) http://www.oost‐vlaanderen.be (22‐04‐’10) http://www.vmm.be/geoview/ (15‐02‐’10) http://www.vmm.be/water/publicaties/biologische‐waterkwaliteit‐in‐vlaanderen‐2007‐kaart (24‐06‐ ‘09)

Plate 1: Freshwater gastropods

Plate 1: A: Lymnaea stagnalis, B: Radix balthica, C: Radix auricularia, D: Acroloxus lacustris, E: Physa fontinalis, F: Valvata piscinalis, G: Galba truncatula, H: Myxas glutinosa, I: Bithynia tentaculata, J: B. tentaculata operculum, K: Planorbis planorbis, L: Anisus vortex, M: Gyraulus albus, N: Hippeutis complanatus, O: Bathyomphalus contortus, P: Gyraulus crista, Q: Valvata cristata. The upper scale bar applies to species A, B and C , the lower scale bar to species P and Q, the middle scale bar applies to all other species.

Plate 2: Freshwater bivalves and terrestrial species

Plate 2: A: Pisidium amnicum, B: Pisidium casertanum, C: Pisidium milium, D: Pisidium nitidum, E: Pisidium obtusale, F: Pisidium pulchellum, G: Pisidium subtruncatum, H: Pisidium tenuilineatum, I: Sphaerium corneum, J: Succinea putris, K: Succinella oblonga, L: Pupilla muscorum, M: Vertigo sp., N: Carychium minimum, O: Vallonia pulchella, P: Punctum pygmaeum. The lower scale bar applies only to Sphaerium corneum, the upper scale bar applies to all other species.

List of appendices O

Appendix 1: Absolute abundance S2

Appendix 2: Absolute abundance S4

Appendix 3: Relative abundance S2

Appendix 4: Relative abundance S4

Appendix 5: Biodiversity indices S2 and S4

Appendix 6: Environmental indices S2 and S4

Appendix 7: Absolute abundance recent material

Appendix 8: Relative abundance recent material

Appendix 9: Environmental parameters

Appendix 10: R protocol (CA + envfit)

Appendix 11: Pollution tolerance ranking (Mouthon & Charvet, 1999)

Appendix 1: Absolute abundance S2

S2 Samples 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 A. lacustris 2 A. vortex 1 B. contortus 1 B. tentaculata 1 66 83 51 62 21 19 1 37 522 624 255 G. trunculata G. albus 5 4 18 7 3 45 75 90 24 6 36 106 4 10 G. crista f. cristata 1 8 5 7 10 22 6 23 170 475 202 81 94 63 184 3 22 H. complanatus 7 1 1 11 40 136 155 66 35 12 46 2 1 L. stagnalis 1 1 1 5 5 3 3 2 M. glutinosa 2 5 1 4 61 10 5 52 73 25 30 11 1 P. fontinalis 1 2 1 17 49 27 19 19 29 1 P. amnicum 1 11 P. casertanum 16 13 40 5 4 14 P. milium 12 1 1 12 57 1 8 4 10 4 1 7 19 3 P. nitidum 1 1 P. obtusale 5 1 25 P. pulchellum 7 2 2 5 1 P. subtruncatum 26 1 10 48 1 12 3 23 1 P. tenuilineatum 1 P. planorbis 1 R. auricularia 1 R. balthica/peregra 2 1 2 2 19 76 71 61 35 34 18 2 2 S. corneum 1 1 1 2 2 1 V. cristata 5 2 1 3 18 18 11 25 2 V. piscinalis 19 1 1 10 66 4 7 19 17 18 16 2 20 45 Aquatic sp. total 1 2 171 102 64 151 366 71 96 425 1459 1223 559 207 206 464 11 41 C. minimum P. pygmaeum P. muscorum S. oblonga S. putris 1 V. pulchella Vertigo sp. Terrestrial sp. total 1 Number of shells 1 2 171 102 64 151 366 71 96 425 1459 1223 559 207 206 464 11 42 Number of species 1 2 11 8 8 16 17 11 13 15 11 11 11 9 9 12 4 9

Appendix 2: Absolute abundance S4

S4 Samples 1 2 3 4 5 6 7 8 9 10 11 12 13 14 A. lacustris 1 A. vortex 1 B. contortus 2 B. tentaculata 19 43 16 61 13 169 685 G. trunculata 1 G. albus 3 8 151 58 66 30 39 23 17 G. crista f. cristata 14 12 2 3 40 196 64 86 68 91 137 51 3 H. complanatus 2 1 13 96 28 60 19 6 11 L. stagnalis 1 2 1 6 1 1 2 M. glutinosa 4 26 4 4 13 18 3 2 P. fontinalis 10 118 17 20 1 P. amnicum 1 5 P. casertanum 2 41 3 5 2 1 3 P. milium 2 40 2 4 1 4 4 2 P. nitidum 1 3 P. obtusale 29 2 P. pulchellum 20 1 P. subtruncatum 5 29 3 5 1 P. tenuilineatum P. planorbis R. auricularia R. balthica/peregra 4 5 26 140 47 62 9 7 20 S. corneum 2 16 6 V. cristata 2 5 8 11 6 27 4 V. piscinalis 24 99 9 61 45 15 7 3 2 Aquatic sp. total 22 110 343 88 47 299 1480 282 319 124 158 189 92 3 C. minimum 2 P. pygmaeum 4 P. muscorum 2 S. oblonga 7 S. putris V. pulchella 4 Vertigo sp. 1 Terrestrial sp. total 20 Number of shells 42 110 343 88 47 299 1480 282 319 124 158 189 92 3 Number of species 9 13 16 9 8 8 13 9 10 4 12 8 6 1

Appendix 3: Relative abundance S2

S2 Samples 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 A. lacustris 0.5 A. vortex 0.7 B. contortus 1.4 B. tentaculata 50.0 38.6 81.4 79.7 41.1 5.7 26.8 1.0 8.7 35.8 51.0 45.6 G. trunculata G. albus 2.9 3.9 11.9 9.9 3.1 10.6 5.1 7.4 4.3 2.9 17.5 22.8 36.4 23.8 G. crista f. cristata 50.0 4.7 4.9 10.9 6.6 6.0 8.5 24.0 40.0 32.6 16.5 14.5 45.4 30.6 39.7 27.3 52.4 H. complanatus 4.1 0.7 0.3 11.5 9.4 9.3 12.7 11.8 16.9 5.8 9.9 18.2 2.4 L. stagnalis 1.6 0.3 0.2 0.4 0.9 1.4 1.5 0.4 M. glutinosa 1.2 4.9 1.6 2.6 16.7 14.1 5.2 12.2 5.0 2.0 5.4 5.3 0.2 P. fontinalis 1.6 1.3 1.0 4.0 3.4 2.2 3.4 9.2 14.1 0.2 P. amnicum 0.7 3.0 P. casertanum 9.4 8.6 10.9 5.2 0.9 3.0 P. milium 7.0 1.0 1.6 7.9 15.6 1.4 8.3 0.9 0.7 0.3 0.2 3.4 4.1 7.1 P. nitidum 0.3 0.2 P. obtusale 2.9 0.7 6.8 P. pulchellum 1.9 2.8 0.5 1.1 2.4 P. subtruncatum 15.2 1.0 6.6 13.1 1.4 12.5 0.7 5.0 2.4 P. tenuilineatum 0.7 P. planorbis 0.1 R. auricularia 0.3 R. balthica/peregra 1.3 0.3 2.8 2.1 4.5 5.2 5.8 10.9 16.9 16.5 3.9 18.2 4.8 S. corneum 100.0 0.3 0.2 1.0 1.0 2.4 V. cristata 2.9 2.0 1.6 2.0 25.4 18.8 2.6 1.7 0.2 V. piscinalis 11.1 1.0 1.6 6.6 18.0 5.6 7.3 4.5 1.2 1.5 2.9 1.0 9.7 9.7 C. minimum P. pygmaeum P. muscorum S. oblonga S. putris 2.4 V. pulchella Vertigo sp. Number of shells 1 2 171 102 64 151 366 71 96 425 1459 1223 559 207 206 464 11 42 Number of species 1 2 11 8 8 16 17 11 13 15 11 11 11 9 9 12 4 9

Appendix 4: Relative abundance S4

S4 Samples 1 2 3 4 5 6 7 8 9 10 11 12 13 14 A. lacustris 1.1 A. vortex 1.1 B. contortus 0.1 B. tentaculata 45.2 39.1 4.7 69.3 27.7 56.5 46.3 G. trunculata 1.1 G. albus 2.7 2.3 10.2 20.6 20.7 24.2 24.7 12.2 18.5 G. crista f. cristata 12.7 3.5 2.3 6.4 13.4 13.2 22.7 27.0 54.8 57.6 72.5 55.4 100.0 H. complanatus 1.8 0.3 4.3 6.5 9.9 18.8 15.3 3.8 5.8 L. stagnalis 0.9 0.6 0.3 0.4 0.3 0.6 1.1 M. glutinosa 3.6 7.6 4.5 8.5 4.3 1.2 1.1 1.3 P. fontinalis 3.3 8.0 6.0 6.3 0.6 P. amnicum 0.9 1.5 P. casertanum 1.8 12.0 3.4 10.6 0.6 0.6 1.6 P. milium 1.8 11.7 2.3 1.4 0.3 2.5 2.1 2.2 P. nitidum 2.4 0.2 P. obtusale 8.5 4.3 P. pulchellum 5.8 1.1 P. subtruncatum 4.5 8.5 3.4 10.6 0.6 P. tenuilineatum P. planorbis R. auricularia R. balthica/peregra 3.6 1.5 8.7 9.5 16.7 19.4 5.7 3.7 21.7 S. corneum 0.6 5.7 1.9 V. cristata 4.8 4.5 2.3 12.5 12.8 9.0 0.3 V. piscinalis 21.8 28.9 19.1 4.1 16.0 4.7 5.6 1.9 1.1 C. minimum 4.8 P. pygmaeum 9.5 P. muscorum 4.8 S. oblonga 16.7 S. putris V. pulchella 9.5 Vertigo sp. 2.4 Number of shells 7 5 2 13 13 9 0 0 0 0 0 0 0 0 Number of species 9 13 16 9 8 8 13 9 10 4 12 8 6 1

Appendix 5: Biodiversity indices S2 and S4

Sequence S2:

Sample 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Shannon‐Wiener 0.00 0.69 1.94 0.80 0.81 2.03 2.23 1.96 2.15 1.97 1.69 1.54 1.71 1.59 Simpson 0.00 0.50 0.80 0.33 0.35 0.79 0.87 0.82 0.86 0.79 0.75 0.69 0.74 0.72 Species richness 1.00 2.00 11.00 8.00 8.00 16.00 17.00 11.00 12.00 15.00 11.00 11.00 11.00 9.00 Evenness NaN 1.00 0.81 0.39 0.39 0.73 0.79 0.82 0.87 0.73 0.70 0.64 0.71 0.72 Sample 15 16 17 18 Shannon‐Wiener 1.85 1.77 1.34 1.46 Simpson 0.81 0.76 0.73 0.66 Species richness 9.00 12.00 4.00 9.00 Evenness 0.84 0.71 0.97 0.66

Sequence S4:

Sample 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Shannon‐Wiener 1.72 1.89 2.28 1.16 1.93 1.43 1.70 1.92 1.77 1.12 1.30 1.02 1.15 0.0 Simpson 0.74 0.77 0.86 0.50 0.83 0.64 0.74 0.84 0.80 0.61 0.60 0.45 0.61 0.00 Species richness 9.00 13.00 16.00 9.00 8.00 8.00 12.00 9.00 10.00 4.00 11.00 8.00 6.00 1.00 Evenness 0.78 0.74 0.82 0.53 0.93 0.69 0.69 0.88 0.77 0.81 0.54 0.49 0.64 NaN

Appendix 6: Environmental indices S2 and S4

Sequence S2:

S2 Drought Size Movement Temperature Sample Depth nD nR nD/(nD+nR) nL nS nL/(nL+nS) nSt nF nSt/(nSt+nF) nW nC nW/(nW+nC) 1 0 100.0 0.0 1.00 0.0 0.0 #DEEL/0! 0.0 0.0 #DEEL/0! 0.0 0.0 #DEEL/0! 2 4 50.0 0.0 1.00 50.0 0.0 1.00 50.0 0.0 1.00 50.0 0.0 1.00 3 9 11.7 12.3 0.49 50.9 9.9 0.84 11.7 7.0 0.63 38.6 14.0 0.73 4 19 5.9 2.0 0.75 87.3 1.0 0.99 11.8 1.0 0.92 81.4 1.0 0.99 5 29 12.5 1.6 0.89 85.9 1.6 0.98 15.6 1.6 0.91 81.3 1.6 0.98 6 39 14.6 11.9 0.55 52.3 8.6 0.86 13.9 9.3 0.60 41.1 7.3 0.85 7 52 22.3 11.3 0.66 45.1 24.5 0.65 31.0 20.6 0.60 6.6 25.0 0.21 8 59 11.3 28.2 0.29 46.5 4.2 0.92 50.7 4.2 0.92 26.8 5.6 0.83 9 63 32.3 26.0 0.55 14.6 8.3 0.64 50.0 8.3 0.86 1.0 7.3 0.13 10 70 41.2 8.0 0.84 29.9 1.4 0.95 59.5 1.4 0.98 8.9 4.5 0.67 11 73 33.2 7.0 0.83 45.3 0.7 0.99 44.5 0.7 0.98 35.8 1.2 0.97 12 82 16.8 6.0 0.74 57.2 0.3 0.99 24.9 0.3 0.99 51.4 1.5 0.97 13 90 14.8 10.9 0.58 58.1 0.2 1.00 31.7 0.2 0.99 46.5 2.9 0.94 14 93 46.4 16.9 0.73 16.9 0.0 1.00 69.1 0.0 1.00 1.4 1.0 0.60 15 104 35.0 16.5 0.68 25.2 3.4 0.88 48.5 3.4 0.93 1.5 9.7 0.13 16 115 43.8 6.9 0.86 10.6 5.2 0.67 44.2 5.2 0.90 0.4 9.7 0.04 17 122 27.3 18.2 0.60 0.0 0.0 #DEEL/0! 45.5 0.0 1.00 0.0 0.0 #DEEL/0! 18 129 61.9 4.8 0.93 0.0 9.5 0.00 57.1 9.5 0.86 0.0 0.0 #DEEL/0!

Sequence S4:

S4 Drought Size Movement Temperature Sample Depth nD nR nD/(nD+nR) nL nS nL/(nL+nS) nSt nF nSt/(nSt+nF) nW nC nW/(nW+nC) 1 0 2.4 4.8 0.33 47.6 0.0 1.00 4.8 0.0 1.00 45.2 0.0 1.00 2 21 14.5 10.0 0.59 66.4 1.8 0.97 25.5 2.7 0.90 40.0 21.8 0.65 3 51 15.7 15.7 0.50 43.1 25.9 0.62 23.9 19.0 0.56 5.2 37.3 0.12 4 67 4.5 15.9 0.22 75.0 2.3 0.97 21.6 2.3 0.90 70.5 0.0 1.00 5 74 6.4 23.4 0.21 55.3 4.3 0.93 31.9 0.0 1.00 27.7 23.4 0.54 6 80 13.4 17.7 0.43 64.5 0.0 1.00 35.8 0.0 1.00 56.9 0.0 1.00 7 96 13.6 9.7 0.58 60.2 0.0 1.00 24.6 0.0 1.00 46.7 4.1 0.92 8 110 29.8 16.7 0.64 23.0 1.4 0.94 40.4 1.4 0.97 0.0 16.0 0.00 9 130 29.2 20.1 0.59 11.3 0.3 0.97 46.7 0.3 0.99 0.3 4.7 0.06 10 133 54.8 0.0 1.00 5.6 0.0 1.00 54.8 0.0 1.00 0.0 5.6 0.00 11 137 60.1 6.3 0.90 4.4 2.5 0.64 65.2 2.5 0.96 0.6 1.9 0.25 12 142 74.6 5.3 0.93 2.1 2.1 0.50 77.2 2.1 0.97 1.1 1.1 0.50 13 147 57.6 22.8 0.72 0.0 3.3 0.00 77.2 3.3 0.96 0.0 0.0 #DEEL/0! 14 157 100.0 0.0 1.00 0.0 0.0 #DEEL/0! 100.0 0.0 1.00 0.0 0.0 #DEEL/0!

Appendix 7: Absolute abundance recent material

Biological water quality G G G G G G G M M B B Samples 38020 39000 39800 44000 52200 52700 52800 40800 53300 53500 53550 Acroloxus lacustris 1 7 9 1 Anisus vortex 2 21 2 10 8 Bathyomphalus contortus 6 Bithynia leachii 14 1 5 5 Bithynia tentaculata 28 144 3 13 212 2 184 187 4 Dreissena polymorpha 2 Gyraulus albus 35 5 Gyraulus crista f. cristata 6 12 Hippeutis complanatus 1 2 2 34 1 Lymnaea palustris 1 Lymnaea sp. 3 1 1 Lymnaea stagnalis 1 Musculium lacustre 3 20 Physa acuta 1 12 1 6 2 12 1 Physa fontinalis 20 1 Pisidium subtruncatum 45 Planorbarius corneus 10 4 19 1 2 Planorbis planorbis 1 12 1 Radix balthica/peregra 28 26 3 10 6 4 9 2 Sphaerium corneum 24 42 55 33 Valvata cristata 20 3 7 Valvata piscinalis 1 61 14 4 Number of molluscs 92 257 12 289 233 8 329 254 27 1 0 Number of species 10 9 5 13 9 4 11 10 5 1 0

Appendix 8: Relative abundance recent material

Biological water quality G G G G G G G M M B B Samples 38020 39000 39800 44000 52200 52700 52800 40800 53300 53500 53550 Acroloxus lacustris 0.4 2.1 3.5 3.7 Anisus vortex 0.8 7.3 0.9 3.9 29.6 Bathyomphalus contortus 2.3 Bithynia leachii 5.4 8.3 1.7 2.1 Bithynia tentaculata 30.4 56.0 25.0 4.5 91.0 25.0 55.9 73.6 14.8 Dreissena polymorpha 2.2 Gyraulus albus 10.6 2.0 Gyraulus crista f. cristata 2.1 3.6 Hippeutis complanatus 1.1 0.7 0.9 10.3 0.4 Lymnaea palustris 0.4 Lymnaea sp. 3.3 0.4 0.4 Lymnaea stagnalis 1.1 Musculium lacustre 3.3 6.1 Physa acuta 1.1 4.7 12.5 1.8 0.8 44.4 100.0 Physa fontinalis 6.9 0.3 Pisidium subtruncatum 15.6 Planorbarius corneus 3.9 33.3 6.6 0.4 0.8 Planorbis planorbis 1.1 4.2 12.5 Radix balthica/peregra 30.4 10.1 25.0 3.5 2.6 50.0 2.7 7.4 Sphaerium corneum 26.1 16.3 19.0 13.0 Valvata cristata 6.9 1.3 2.1 Valvata piscinalis 8.3 21.1 4.3 1.6 Number of molluscs 92 257 12 289 233 8 329 254 27 1 0 Number of species 10 9 5 13 9 4 11 10 5 1 0

Appendix 9: Environmental parameters

Parameter Unit 38020 39000 39800 44000 52200 52700 52800 40800 53300 53500 53550 WQ ‐ 7.5 7.5 7.5 7.5 7.5 7.5 7.5 5.5. 5.5 3.5 3.5 Cl mg/L 58.40 44.70 49.60 42.80 72.40 NA NA 157.00 35.50 43.00 56.00 N3 mgN/L 0.88 0.67 1.10 <0,40 1.20 NA NA 0.72 3.50 1.90 <0,23 N2 mgN/L 0.19 0.15 0.17 <0,02 0.09 NA NA NA 0.36 0.34 0.14 PO mgP/L 0.27 0.43 0.29 0.24 0.00 NA NA 0.10 0.47 0.99 1.50 SO mg/L 58.20 NA 54.20 65.20 <0,14 NA NA NA 85.10 NA NA Pt mgP/L 0.67 0.64 0.66 0.32 <0,14 NA NA NA 0.78 1.10 2.20 Ov % 56.00 28.50 19.50 40.40 160.10 120.80 48.30 46.30 38.80 72.10 67.80 EC µS/cm 579.00 492.00 550.00 613.00 886.00 1782.00 1046.00 1025.00 667.00 703.00 750.00 O2 mg/L 5.36 2.94 1.80 4.02 14.32 11.88 4.84 4.50 3.41 6.71 6.35 pH ‐ 7.61 7.48 7.45 7.18 8.21 8.28 7.97 7.70 7.59 7.78 7.93 ZS mg/L 32.00 29.50 16.00 <1,6 14.80 NA NA NA 9.50 8.80 22.00 NH mgN/L 1.70 2.70 2.70 0.26 0.35 NA NA 0.16 4.00 8.20 11.00 KN mgN/L 3.60 3.80 3.90 <0,90 2.10 NA NA NA 5.20 7.70 14.00 BZ mgO2/L 2.80 3.90 4.70 <0,50 6.80 NA NA NA 4.50 3.50 8.70 CZV mgO2/L 33.30 36.70 29.50 23.60 45.90 NA NA NA 20.20 35.00 55.00

Appendix 10: R protocol (CA + envfit) library(vegan) a=read.table("abundantieII.txt",header=TRUE) show(a) o=read.table("omgevingII.txt",header=TRUE) show(o) w=decorana(a) show(w) y=cca(a) summary(y) plot(y) x <‐ envfit(y,o, permutations=10000, na.rm=TRUE) plot (x, add = TRUE) show(x)

Appendix 11: Pollution tolerance ranking (Mouton & Charvet, 1999)